I recently tried RIT fabric dye on some durable plastic 3d printed ponoko mascots; so here's a few photos of the process and the colorful results. The dyes I used were a powdered 'royal blue' and a liquid 'cherry red'.
To start things off I took a small pot I don't cook with and heated up some tap water on the kitchen stove. Brought the water to a near boil then backed it down to a simmer. Adding the dye was a wholly unscientific process for me, only a small amount of the packet was needed to get a deep blue hue.
Stirring well I made sure the dye granules had dissolved completely before putting my model in.
No turning back now...
This little guy wanted to float; felt a little macabre as I forced it under and bubbles gurgled out the mouth...
A series of shots at 10min, 20 minutes and 30 minutes. Your results will vary depending on how much dye you use and how hot you keep the water. If you want a repeatable color take detailed notes on the amount of water, water temperature and the amount of dye added in addition to soak time.
Around the 30 minute mark it reached the color I was looking for so I pulled it out and put it under the kitchen faucet to wash off any remaining dye. Initially the model looked blotchy but as it dried took on a much more uniform color.
The liquid RIT dye performed like the powdered version, although I perfered the liquid when it came to being able to recap and not getting dye granules where they shouldn't be.
Depending on how long you leave a model in you could achieve everything from a pale pink to a dark red.
What a cheerful group.
Here's one more example of the white durable plastic dyed cherry red.
Have you tried your hand at dying dying your own models? If so how did it turn out?