Fill a pot with water enough to submerge largest piece, and heat to just shy of deforming the plastic (use scrap to test). Add Ritt dye (1 pkg per gal) and salt (a few tablespoons per gallon). The salt is a very important ingredient that helps the dye take!. Submerge pieces in solution for a minute or so. Rinse with cool water and check density. Repeat until desired darkness is acheived.
DON"T FORGET THE SALT!