resin - bonded fixed partial dentures ( fpds ) are inexpensive and there are more conservative treatment options for replacing missing teeth [ 1 , 2 ] . metal resin - bonded fpds continue to decrease demand because of the existing several disadvantages . while metal substrate is durable and stiff , it has considerable esthetic problems such as discoloration of the gingiva and abutment tooth  graying. the other drawbacks may occur with these prostheses ; namely , the potential for alloy hypersensitivity , retainer fracture , and loss of attachment from bridge abutments , corrosion and health risks to laboratory staff [ 35 ] . given thes problems , composite materials have been a subject to attention for metal - free fpds after the advent of using fibers as frameworks to reinforce them . in addition , these economically feasible restorations have proper esthetics , low weight and favorable elastic modulus [ 2 , 4 , 6 ] . conversely , color stability and wear resistance are remarkable potentials of ceramics , but these materials have some liabilities such as brittleness , disability of adhesion to tooth structure and having the potential to damage the unrestored opposing teeth [ 3 , 7 ] . based on these backgrounds , currently frc are not only used for crowns and inlays , but also for a variety of esthetic restorations in clinics , for example the suprastructure of implants and fpds . different kinds of fiber materials exist in the market , but at present polyethylene and glass fibers are the most popular . to date , several studies have evaluated the mechanical properties ( flexural strength and fracture strength ) of frc [ 4 , 6 , 8 , 10 ] , but a few studies have been conducted to find out and understand the color properties of frc [ 1114 ] . .   stated significant color change between non - frc and frc restorations with a decrease in lightness . tuncdemir and aykent   measured the effect of fiber reinforcement on the color stability of composite resin . they concluded that an everstick net fiber - reinforced anterior composite combination exhibits trace color change , but the other frc groups reveal slight color change without accelerated aging . after accelerated aging , they found no significant color difference between frc and non - frc restorations . these studies analyze different types of composites with varying types of fibers and the results are different [ 1114 ] . so , this study investigated the influence of glass and polyethylene fibers and veneering composites ( direct and indirect ) in the frc system on color differences . the null hypothesis assumed in this study was that the fibers and veneering composites did not influence the color differences of fiber - reinforced composites and non - fiber - reinforced composites . two types of fiber - reinforced system and veneering composite were investigated in the present study ( table 1 ) . specimen preparation : using two stainless steel molds , 13 mm in diameter and 1 mm or 3 mm height , 40 cylindrical specimens were prepared ( fig 1 ) . composite resin veneers were packed into a mold on a glass plate . after packing , a second glass plate was placed over the mold , followed by compression to run off the excess composite resin into the escape area . the z250 cylinders were light - cured using the light curing unit ( bluephase ivoclar vivadent , austria ) for 20s with an output of 1,000 mw / cm2 light intensity . the output of the curing light was checked with a radiometer ( 1,000 mw / cm2 ) . but the initial curing of gradia specimens was done according to the recommendations by gc steplight sl - i for 10s . after light - curing , the top glass plate was removed and the specimens were then removed from the mold . twenty ( 1 mm height ) cylinders of each base composite were prepared according to the manufacturer`s recommendations ( 1618 ) . a 20-second duration cure and handling of glass slab was similar to the veneering composite . these cylinders were entered in the thicker mold . in the cylinders of nulite f base composites , polyethylene fibers were cut in 10 mm size , impregnated in resist resin and embedded on thin layers of nulite f composites and post - cured 20 seconds after , then a small layer of nulite f composite covered the fiber . subsequently , the cylinders of each veneering composite material were carried to a thicker mold and pressed with a glass slab to remove the excess composite . in z250 veneering composite , the specimens were light cured using the blue phase ( ivoclar vivadent , austria ) visible - light - curing unit for 20s with 1000mw / cm2 light intensity ( g1 ) and gradia indirect completion of curing was done in gc labolight lv- ,  cure unit ( g3 ) . in adhesive c cylinders , the proper pieces ( 10 mm ) of glass fibers were cut , placed on a thin layer of adhesive c composite and cured for 20 seconds . veneering composites in ( g5 ) and ( g7 ) were used in a similar manner as explained above for ( g1 ) and ( g3 ) , respectively . in g2 and g6 , z250 cylinders were placed in thicker molds and the remaining spaces in the molds were packed with nulite f and adhesive c , respectively . for g4 and g8 , gradia indirect cylinders were placed in thicker molds and the remaining spaces in the molds were packed with nulite f and adhesive c , respectively . the color parameters of the disks were measured over a white background using a reflection spectrophotometer ( color - eye 7000a , gretagmacbeth , new windsor , ny , usa ) , according to cie l * a * b * color system relative to the standard illumination tungsten d65 lamp . prior to each of the color measurement series , a calibration was done based on the manufacturers instruction . here , a * , b * represented the colors on the green - red and blue - yellow axes , respectively and l * represented lightness scale . the color differences ( e ) between the groups were calculated by equation : e * = [ ( l*)2 + ( a*)2 + ( b*)2 ] 1/2 . the extent of significant color difference was evaluated by two - way analysis of variance ( anova ) combined with tukey s hsd at the 0.05 level of significance . the color parameters of the disks were measured over a white background using a reflection spectrophotometer ( color - eye 7000a , gretagmacbeth , new windsor , ny , usa ) , according to cie l * a * b * color system relative to the standard illumination tungsten d65 lamp . prior to each of the color measurement series , here , a * , b * represented the colors on the green - red and blue - yellow axes , respectively and l * represented lightness scale . the color differences ( e ) between the groups were calculated by equation : e * = [ ( l*)2 + ( a*)2 + ( b*)2 ] 1/2 . the extent of significant color difference was evaluated by two - way analysis of variance ( anova ) combined with tukey s hsd at the 0.05 level of significance . the values of l * , a * , b * , and the differences in color ( e ) are indicated in table 3 . application of two - way anova revealed that l * , a * and b * were significantly affected by the veneering composites ( p<0.05 ) , and by the fibers ( p<0.05 ) . the negative sign of l * shows that incorporation of fibers into composite in experimental groups made them darker than the controls , except in g7 . g1 exhibited a red - blue shift ( a*= 0.98 , b*= 0.93 and the other groups ( g3 : b*= 1.54 , a*= 0.37 , g5 : b*= 1.82 , a*= 0.42 , g7 : b*= 0.77 , a*= 0.61 ) a green - blue shift . pairwise comparison by tukey s hsd test confirmed the significant statistical differences in a * and l * between g1 and g3 , between g1 and g5 and between g3 and g7 ( p<0.05 ) . furthermore , pairwise comparison by tukey s hsd test indicated no significant statistical differences in b * between g1 and g3 and between g3 and g7 ( p<0.05 . tukey s hsd indicated statistical differences in b * between g1 and g5 ( p=0.004 ) . the null hypothesis was verified because no significant differences between the groups studied in e were observed . e values of all groups vary from 1.91 to 2.32 , which were lower than the threshold of clinical acceptable color difference . clinical color - matching tolerance between the tooth and restorations may be categorized according to e , based on clinical studies below : ( e:0=perfect , e:0.51=excellent , e:12= good , e:23.5=clinically acceptable , e :> 3.5=mismatch ) . according to these classifications , clinical color match in g3 and these results were in disagreement with the findings of a previous study , whereby incorporation of fibers in the composite caused color mismatches that were not clinically acceptable ( 5.29 < e < 8.19 ) . they only investigated the incorporation of glass - fiber in single - layer and double - layer in direct dpi curex microhybrid composite ( shade a1 ) and the overall thickness of all specimens were 2 mm . one possible explanation about the different results is that the greater amount of veneering composite for covering specimens may reduce the effect of fibers . since the type of fibers and veneering composites were clearly different from the current study , direct comparison was impossible . tuncdemir and aykent   demonstrated that the types of composite and fiber materials used explained the color difference between frc and non - frc restorations without aging . they have reported less color differences ( 0.32<e<1.03 ) between groups than those observed ( 1.91<e<2.32 ) in the present study . differences in the range of e in our study and the previous study   may be due to the use of these base composites . .   showed that reproduction of the similar color to the veneering composite was possible in all experimental groups ( except in the case of fibrekor ) . as indicated in earlier studies , the key factors affecting the optical properties ( color change ) in composites are as follows : ( i ) size , shape and filler content , ( ii ) type of polymerization , ( iii ) organic matrix , and ( iv ) thickness of composite . z250 is a microhybrid composite that contains bis - gma , udma and bisema monomers and zirconium / silicon fillers ( 60% volume , 0.01 to 3.5 micrometers ) . gradia is a micro - filled udma - based composite that is mixed with a fine particle glass filler , silica nanofiller and prepolymerized filler ( 75 wt % ) . although compositions and the method of polymerization of the studied composites varied , these differences had no significant effect on color changes . these results could be linked to the interaction between fibers and composites . on the other hand , this in vitro finding may be different from the clinical circumstances . meanwhile , all specimens employed as the model of color assessment in this study were flat and this led to the omission of surface morphology - associated color characteristics . in this study , the comparison between the observations of the human eyes or conventional methods showed that spectrophotometer offered a 33% enhancement in precision . in the present study , the visual inspection of skillful people and other viewers did not verify the results of the spectrophotometer . conversely , one review highlighted that visual and instrumental methods completed each other and could serve as a guide towards predictable cosmetic results . thus , evaluation of other color coordinates is essential . with respect to the values of l , the incorporation of fibers caused a decrease of the l * values ( samples became darker ) for all the experimental groups except in g7 . more negative l values between the polyethylene experimental groups and the glass experimental groups possibly originated from the greater thickness of the polyethylene fibers , and also from the difference in the nature of fibers between these two types of fibers , which could result in a slighter amount of beam reflected back ; back - scattered out of the specimen or absorbed . in the present state of our knowledge , we did not find any justification for the result in g7 . there is a color shift of experimental groups towards the shorter wavelength regions , which is in the direction of green and blue , except in g1 . these findings are inconsistent with the previous study because they saw a shift toward the longer wavelength ( red - yellow ) . perhaps the interactions between fibers and veneering composite in all experimental groups ( except in g1 ) were followed by shifts toward the lower wavelength area . on the basis of the limited information on the interaction modality of the optical properties of the fiber and the veneering composite in literature , we did not suggest any explanation responsible for the shift to the red axis in g1 . it should be kept in mind that the result of the present article is based on an in vitro condition . meanwhile , this study only investigated the combination application of the two types of fibers and veneering composites . therefore , it is intricate to generalize the outcome of this study to all frc systems and clinical conditions . the clinical conditions may worsen the status of the continuing test and may affect the results . several variations exist between clinic and test conditions that may influence the color change of frcs including : ( i ) the investigated specimens had flat surfaces ; whereas , in clinical status the anatomical surfaces were reproduced on prosthesis , ( ii ) the preparation of the abutment teeth , ( iii ) oral conditions ( loading , temperature changes , saliva , diet and oral hygiene ) . consequently , a more complete strategy should be developed to assess the oral environment effects on color changes of frcs . with the limitations of the current study , veneering composites and the fibers showed no statistical effect on the color change ( e ) of frcs . all experimental groups were darker compared with the control groups except for the glass fibers - gradia indirect group .