Some colors must be added cold. Typically they are the clear Fluorescent colors. They will clump and string if added hot. Almost all others can be added either hot or cold. If I am working on a new color I will add after heating to get a better look see. If it is a known recipe I typically add my color to cold plastic. I always add glitter after the catalyst heat up ( 350 ) and stir. Most colors are a little more vibrant if added cold. Dad gum, Denny and Ctom are fast. ( and accurate )