Hi Buddy,
Like Bob above, I dye by adding small amounts of dye little by little until the color is what is wanted. The material is checked from time to time by taking a bit out, rinsing well, and then drying it with my wife's hair drier.
Normally most of my dying is to save money. This is done by dying such things as rabbit, hen saddle, soft hackle, and bugger hackle. I tie buggers, but not tons of them. Thus, for example on the bugger hackle, I bought a couple of white bugger saddles, and then dye some feathers from them brown, yellow, orange, black (pretty close any way), and chartreuse. Since buying one or two saddles is much less expensive than buying a half dozen, and the same for hen saddles and soft hackle, it actually does save money.
I also dye to get feathers or fur that is otherwise very hard to come up with for certain pattern. For example, I have dyed grizzly hackle green for tying green drakes. Further, it gets pretty expensive to buy a whole package of hackle or soft hackle feathers of a particular color just to tie a few flies from, so I dye my own.
I have also dyed white rabbit several colors to make my own dry fly dubbing. Since I use the same dyes for all types of dying, and use only inexpensive dye (ie: Rit and Kool-aid only), the dye has not cost very much.
However, a few years ago I was able to buy some Zonker strip scraps from an outfit that makes Zonker strips, and I use those for making dubbing and for tailing small buggers. It is much preferable, for me anyway, to use that for dubbing rather than to dye my own. They don't sell the scraps any more, but I wish that they did, because they weren't too expensive, and having them saves dying my own.
Finally I don't try to mix colors much, but rather use most of the dye "as is" to get single colors, so almost never ruin any material.
For me dying is kind of a mess, and I would rather do something else, so I don't do much, but there are reasons for it, so every now and then it is worth doing.
Regards,
Gandolf