You know, I thought about doing this too. It seems possible and might be worthwhile if you mixed up a custom color you planned on using again in the future. It would also save you a little bit of money and time spent on trips to the store.

However, RIT is really cheap and, when using it, I rarely use the entire packet/bottle. For custom colors you could just jot down the "recipe."

I guess it just depends on whether or not you feel like storing bottles/jars of the dye solution.

Looking at it from the opposite end; does anybody know how to bleach vinyl?

I have a question for anyone who died toys a year or so ago.
How have they been for colorfastness? I'm guessing since it appears to soak right into the vinyl that it is fairly good, but I was wondering if it's more sensitive to sun exposure?