both the white chocolate and candy melts have more fat in them than semi-sweet. when making these clays, you should be using a recipe that uses less corn syrup than when making with semi-sweet and/or absorb some of the fat off before using.
toba garrett (my cake hero) says she uses unused newprint paper to absorb the fat, but i never have this laying around
. what i do is to wipe the fat off after i have wrapped it well and aged it in my fridge for 24 hours (it goes in the fridge while still warm and soft). when i take it out, i give it the old once over with a paper towel. kinda yucky
, but the end result is much better for the reduction in fats that make it too soft and oily.
i love working with semi-sweet because you just don't have this issue, but you do only get the one color, lol.
for coloring, the most reliable way is to use oil-based candy colors. however, there are a number of other ways that folks have successfully used, but i cannot speak to those personally.