Mix 5 ml ( teaspoon) colour pigment (colourant) with 200 ml acetone. The pigment available in hardware shops for colouring of paint. Add same amount of plastic pellets to dissolve in the acetone. When fully dissolved spread out on glass and let it dry for a day or two. Cut up in small pieces the same size as yor plastic pellets. Mix at 2-5 % ratio with your plastic pellets and extrude.