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For coloring bath salts (which are a cosmetic) you would need to use a color additive that is approved by the FDA for use in cosmetics. There are lots of color additives that are approved for cosmetics,  however some of them do not work well in bath salts.

Ultramarines are basically made by heat treating kaolin clay and sulphur. At low pH’s (which is the range where bath salts are) ultramarines tend to break down and release sulphur giving off that lovely rotten egg smell. Not a very nice surprise for your customers. While these stink, LOL, they do
not fade.

Iron Oxides will work, but may also leave a residue in the tub. The good thing is they don’t fade.

Micas will work. Some will fade and some won’t. They will most likely leave a residue in the tub.

There are also natural colorants on the FDA’s approved colorant list. These are annatto, caramel, carmine and beta-carotene. There are a few more, but nothing that you would probably want to use in salts. These will all fade in sunlight or UV rays. There are no flowers (like blue malva - which will make water a lovely blue — too bad it does not hold up in soap) or chlorophyll listed on the approved list and using these “simply for the purpose of coloring” your salts would make them an adulterated product, you don’t want that.

Dyes offer the biggest color choice of all. You want to make sure that what you are using in on the approved list for cosmetics. And as with natural color additives, eventually dyes will eventually fade, particularly if exposed to UV rays. There are a couple dyes that are limited in the amount you can use in a product, Green 8 and Red 36, but generally you would have to add so much of these dyes to a product that the product would start coloring other items before you reached that limit.

Unless you are familiar with the ingredients in certain food colorants it is not a good idea  to use them in bath salts or other toiletries.  Some food colorants, particularly gel/paste food colorants, contain Blue#2 and Red#3.  Both Blue#2 and Red#3 are not approved by the FDA for use in cosmetics.

The link to the FDA’s list of approved color additives is: http://www.cfsan.fda.gov/~dms/opa-col2.html 
You can look up the individual colors in 21 CFR part 73, subpart C and 21CFR part 74, subpart C.  Unless you are making make-up or lip stick and eyeshadow, you don’t really need to worrying about what you use as long as it is on the approved list for cosmetics.

I have been asked about the glycerin content of soap made with various oils.  Here is some information that might be helpful in figuring this out. 

First of all the glycerin recovered from commercial soaps is 80% (that does not mean that soap contains 80% glycerin, but that the recovered glycerin isn’t exactly pure.)  After glycerin is recovered from commercial soaps, it is distilled into a purer form.

Okay, that said, here is a list of the available percentages of 80% glycerin contributed to soap by some of the different oils which may be used in the making soap making process according to Modern Soap and Glycerin Manufacture, by E.T. Webb.

18% - Coconut
18% - Palm Kernel Oil
13% - Cottonseed Oil
13% - Soy Bean Oil
13% - Olive Oil
13% - Lard
12.5% - Tallow
12.5% - Castor Oil
12% - Palm Oil (bleached)

When figuring out the percentage of glycerin in your soap you will also have to take into account the percentage of lye, water, and any other additives like herbs, color or fragrance used in the recipe and some how figure how much water evaporates from your soap.

You can figure out how much water evaporates by weighing your soap at the end of the cure period.  Water weighs the same in volume as it does in weight, so 4 liquid ounces of water will weigh 4 ounces.  Whatever amount of weight your soap loses during the cure period, you will have to subtract that from you recipe to get the correct percentages for the ingredients of the recipe or formula.

Sorry, you’ll have to do your own math. 

Also remember that these numbers will simply give you an idea of how much glycerin is in a particular soap.  Just like saponification numbers of various oils, glycerin percentages will also vary from crop to crop depending on growing conditions and other things.   So, your milage may vary.