single or multiple colorants are infused into a thermoplastic material during melt processing to form preforms and plastic containers exhibiting a marbleized or swirl - type pattern . a base resin and colorant may be fed into an extruder having an extrusion screw and / or barrel modified in a manner to cause incomplete and non - uniform mixing . the modifications may include notches in some of the flights along the extrusion screw , a reduction in the depth or omission of some of the flights , a reduction in diameter of the extrusion screw , or the provision of a marbleizing torpedo near the discharge nozzle . other techniques include introducing the colorant into vents present along the barrel , using a barrier screw to keep the components separate along the length of the barrel , using a high pressure pump for introducing the colorant near the discharge nozzle , or using multi - material co - injection techniques for co - injecting the base resin and colorant .