Hi Chapples,
It would be best to order the coloring from your supplier of choice.
As Shane said, Crayons will work but the colors will be all over the place in terms of consistancy..
My recommendation is to get anywhere from 6-10 colors to start..
Black,Brown,Purple,Watermelon, natural/nightcrawler,Green Pumpkin, etc and practice with tints and shades to get the colors you want.
Get some assortments of flake also.. (Black, red,blue,etc in the following sizes to start .015,.035)
1. Never use waterbased scents in your hot plastic. (Anything that you can buy in the store) The plastic will explode and you won't like it.
2. Pour in a well ventilated area and wear a respirator.
3. Pay attention to your plastic. A little heat vapor is okay, smoke is not. If you over heat your plastic it will effect your color and it will eventually scorch.
Whenever you have a question or a concern, post it on TU and you will get the help you need..