1. A method for classifying articles according to their color comprising steps of: selecting a first pair of wavelengths on a plurality of light reflection curves in terms of a reflected light wavelength spectrum; illuminating with light comprising said first pair of wavelengths an area of one of said articles; measuring light reflected from said area of one of said articles at said first pair of wavelengths to obtain detected light reflection values corresponding to said first pair of wavelengths; producing a primary signal representing a resulting difference between said detected light reflection values, said signal being indicative of the color of said one of said articles; selecting at least one secondary wavelength on said reflected light wavelength spectrum; illuminating with light comprising said at least one secondary wavelength said area of said one of said articles; measuring light reflected from said area at said at least one secondary wavelength to produce a secondary signal; said secondary signal being further indicative of the color of said one of said articles; providing a classification frame comprising a pattern consisting of first and second crossing sets of lines respectively defined by mathematical analysis, said classification frame defining a plurality of color classes; classifying said one of said articles in a color class of said plurality of color classes defined by said frame, whenever said primary and secondary signals comply with a specific condition; and rejecting said one of said articles whenever said primary and secondary signals do not comply with said specific condition; and wherein said first and second crossing sets of lines are respectively defined by: EQU R2.sub.i =m.sub.1 (R1.sub.i -R1.sub.0i)+R2.sub.0i and EQU R2.sub.j 32 -m.sub.2 (R1.sub.0j -R1.sub.0j)+R2.sub.0j wherein R1.sub.i is a threshold value for said primary signal on a first axis corresponding to a line i of said first set of lines, i=1,n and n&gt;2; R1.sub.j is a threshold value for said primary signal on a first axis corresponding to a line j of said first set of lines, j=1,m and m&gt;2; R1.sub.0i is the origin threshold value for said primary signal on said first axis corresponding to the line i of said first set of lines; R1.sub.0j is the origin threshold value for said primary signal on said first axis corresponding to the line j of said first set of lines; R2.sub.i is a threshold value for said secondary signal on a second axis corresponding to a line i of said second set of lines, i=1,n and n&gt;2; R2.sub.j is a threshold value for said secondary signal on a second axis corresponding to a line j of said second set of lines, j=1,m and m&gt;2; R2.sub.0i is the origin threshold value for said secondary signal on said second axis corresponding to the line i of said second set of lines; R2.sub.0j is the origin threshold value for said secondary signal on said second axis corresponding to the line j of said second set of lines; m.sub.1 is a slope parameter for the line i of said first set of lines, and being a positive value; m.sub.2 is a slope parameter for the line j of said second set of lines, and being a positive value, and wherein R1.sub.0i=1 and R2.sub.0i=1 are respectively equal to R1.sub.0j=1 and R2.sub.0j=1 ; and wherein said specific condition is defined by: ##EQU3## wherein C.sub.ij is the class delimited by the line i and a line i+1 of said first set of lines and the line j and a line j+1 of said second set of lines, and wherein i&lt;n-1, j&lt;m-1; r1 is said primary signal indicative of the color of said one of said articles; and r2 is said secondary signal further indicative of the color of said one of said articles.