community of dyers

If the dye has completely finished reacting, so that every single dye molecule has reacted either with the fiber or with the water—this means letting it react for longer than would otherwise be necessary—then it is safe to rinse all colors together, assuming that only normal amounts of dye were used (instead of a huge excess), and assuming that the water's not hard or that you're using sodium hexametaphosphate as a water softener.

For tie-dyeing I always do this. I leave the dyed items to react all day and all night, making sure that they stay moist the whole time (whether by including urea or by wrapping in plastic or just by working on a very humid day), and that they're in a warm place (summertime heat is fine). Then I dump all of the dyed items directly into a washing machine that is already full of water and a little Synthrapol, together, regardless of color. I do one hot rinse, then turn off the cool water supply to the washer (since otherwise my washer stupidly adds cold water to the hot cycle, with the aim of saving energy, when if I wanted warm water instead of hot water I would use the warm water setting) and do a couple of washes in very hot water. I wouldn't put the waxed items into the washer, though; I'd want to stick to doing them by hand, since I am worried about wax depositing in the pipes (as has necessitated an expensive repair for at least one dyer). Would combining several items for your hand rinsing and washing save that much effort?

If you're using an excess of dye, or you're not leaving the dye to react on the fabric for far longer than is otherwise necessary, or if your water is hard, doing the initial rinse together is probably not a good approach. If not all of the dye has reacted fully, then there will be some still-reactive dye left, which may permanently fix in places you don't want it to be. It's only oif every single bit of dye has reacted that you can rely on being able to wash out unwanted backstaining. If there's a huge excess of dye, that in itself makes washing out become a pain. And if your water is hard, then some of the Procion dyes form dimers, that is, associations of two dye molecules, which, since they are larger, have a higher degree of substantivity (defined as the tendency of dye to stick to fabric in spite of not reacting with it), which makes them harder to wash out. It's a good idea to use some sodium hexametaphosphate in your washing cycles, not only in your dye mixes, if you tend to have problems with somewhat hard water.

As far as choosing between thickened black or brown dye versus paint: only because you are already using the paint, that may be the least risky choice, simply because you are already very familiar with it. Every new material or method requires some experimentation before you use it on your important projects.

I like dye painting with thickened dye simply because the dye leaves no feeling on the surface of the fabric at all, whereas all fabric paint can be detected by touching, and because since dye penetrates all the way inside each fiber, it lasts longer than fabric paint, which only coats the outside of each fiber and is therefore more subject to wear. You already know what the fabric paint you're using feels like, though, since you've been using it already, and you know whether it wears acceptably well. (Always turn fabric-painted garments inside-out for laundering, and consider always placing each one in a mesh lingerie washing bag in the washer for added protection against wear.)

-Paula