My experience has been that RIT powder works great on PBT, but that the liquid is all but useless on plastic.
Black was awesome but all the other dark colors I tried came out muddy and uneven.
Bright colors like scarlet and sunshine orange were particularly nice.
I have tried to dye ABS more than once, with a few pitiful successes and a lot of failures.
The effective temperature of the dye may be a few scant degrees below than the soften/melt temperature of ABS, but the range and tolerance is very small. There is no point in mentioning numbers, because your results will vary according to your equipment. I will say for certain that it is well below the boiling point.
You can only trust trial and error. Go to a thrift store and buy some $1 junk keyboards for experimentation.