What I am seeing about dying ABS with RIT uses acetone to get the plastic to take it.  With melting pellets, the dye would probably mix in.  My questions would be, how do you distribute the powder evenly throughout the pellets in the hopper and is there enough mixing in the melt zone to spread it out?  The powder may stick to the pellets well enough that the mixture isn't a problem.  If you can give the pellets a static charge somehow, that might help.  It would take a lot of experimentation to figure out concentrations.

Since RIT has been shown to work really well with Nylon, it would be interesting to see how it would work to mix RIT powder with the Nylon powder.

The powder alone I do not believe will work. you actually need the hydrolytic action of the water tobe able to dye the nylon. on a side not coolaid will also work to dye nylon

Since RIT has been shown to work really well with Nylon, it would be interesting to see how it would work to mix RIT powder with the Nylon powder.

I've seen articles about dying ABS with Rit so I would assume it would act as a colorant. Maybe it doesn't and the temperature of the extrusion would kill the color. Who knows?
My thinking was more along the what is possible lines. I know food coloring gets used in all sorts of things it isn't supposed to and can handle oven temperatures so there must be some inks or paints that would blend in to abs in a permanent way.
I would guess that the manufactures of the color pellets use some due in the first place to make them.

Hey, if it doesn't work at all then it was a daft idea.
It just seems wrong to waste so much filament whilst changing color. I know it doesn't cost that much and you could probably reuse some of the partial color to make a darker variant but even so.

The reason I ask is I see there are various colors to add and mixing has come up. (DePartedPrinter is doing a bang up job with all that)
With a liquid you could inject it further down the tube and even use a stepper driver to control the flow. This would mean less filament used when changing colors and the ability to blend your own colors before hand to a more exact standard.
If you could get an injection point just before the nozzle you could swap colors every few feet. Probably would need some clever mixing nozzle at that point though.
Multi Injection points could give CMYK control, but I think I'm getting ahead of myself there.

As I still haven't got round to building a filament extruder yet I can't test anything but I would be interested to know peoples thoughts.