According to their website, salts, essential oils, mud, etc. are fine - they even sell a line of these for use iin their tubs.

I don't bother coloring mine - I put them in colored jars instead. :-) (Actually, for our personal use, they're in a tightly lidded plastic tub under the sink. LOL) Or sometimes I added salts that had naturally occurring colors of their own depending on where they were produced, like the Hawaiian pink, Celtic gray, or Indian "black" salts. I've never used things like artificial dyes. I tend to be more of a minimalist to begin with, preferring my products to be in a more natural state - if I thought something really needed a color, I tended to use a (normally) non-irritating herb that would produce that color. For example, for a yellow-orange citrus-scented soap, I'd add some turmeric or annatto seed, or infuse herbs like rose madder or alkanet root (tricky, as it can stain permanently) into the oil mixture for a pink lotion. But, of course, you can do whatever you like. :-)

jennie, (I am only a cold process soap guy, but I have dabbled in the bath salts) You are only adding a few drops of food colouring per pound of Epsom salts or other salts. I would guess about 5 drops for a pastel colour, add till you get the colour you like. Good luck.

I don't use a recipe. I like several things in mine and the rest is up to my mood, or what's in the ingredient cupboard. I like sea salt and epsom salt. I also like to mix in a bit of collidial oatmeal. (Aveeno~but Walmart has a knock off that is much cheaper, and I use that.) Sometimes I'll grate in some "butter", or mix in a bit of good "skin" oil. Glycerin is a nice medium to mix in the scent and color. But if a person has trouble with yeast infections, then adding "sugar" to their bath is not a good idea. In that case just use a nice oil. Jojoba, rice bran, safflower. Stuff like that. Powdered milk is a nice touch, powdered goat milk too...Really the sky is the limit. Just play, have fun, and let everyone enjoy the good stuff!