The above product says that, per 1 tbsp (16.9 grams) of clear PCL plastic, to use either 5 red pellets, or 5 yellow pellets, or 5 blue pellets, or 8 black pellets. I wish I could measure this by weight, but 8 pellets weigh less than a gram, so my scale can't measure it. I would probably need a centigram scale or milligram scale to handle this. Alternatively, some object-counting software could be used.
You can also start out with general-purpose art-store pigments or dyes (we've used powdered pigments), and blend them in that way. Whenever we've done this, it was an incredibly long and tedious process.
Some things that we found worked well for us:
Wearing gloves protects your hands from being stained, and we found nitrile gloves to be the best at reducing stickiness. (Vinyl gloves were the worst) But even with nitrile gloves, the plastic was still pretty sticky, so it's best to use metal silverware for manipulation whenever possible.
There are several options for heating — boiling, microwaving, and heat gun. We found microwaving gave us the most precise and even heat control. I know that microwaves have intense hot-spots, but I swear to you, it produced more even heating.
There's a trade-off between being hot enough to be minimally pliable, but not so hot that it became too sticky to be able to manipulate. Find the length of time to microwave it time that reaches this "sweet spot". For the amount of plastic shown below, it was 5 seconds.
We found that re-heating less intensely and more frequently ended up being the fastest overall.
The glass plates can be bought by searching for the term "glass cutting boards".
The bowl was made of silicone, which again reduced problems with stickiness.