What did you use to color the plastic and when did you add it?
Food coloring is actually just fine! I’d also encourage you to experiment with different things. All kinds of dyes should be fine.
You should add it during the stage when you have all of the ingredients in liquid form in the pot or pan. I would add it after you have skimmed off any foam or bits that haven’t dissolved, but before you take it off of the heat so that it starts to cool and gel.