RIT DYE....liquid form. can be found at grocery stores, craft stores. What I do is heat up water to just before boiling point, add dye ...up to 3/4 bottle..stir WELL. ( Add a tablespoon of white vinegar as well..it opens up the pores of the plastic better supposedly.)...submerge parts in solution...the longer you keep the parts in the solution, the darker they will be. I stir solution every 20 mins. or so as well. I generally leave things in for up to 1 hr. depending on what I am doing.
Good luck! A bottle is about $4-5
Here are 2 pics. of rims that I have done...
1. MBX-6 T8 1900KV, RX8 ON 4S
2. MBX-5T 1520, MMM ON 5S
3. MBX-5 ONROAD CONVERSION 1515, MMM ON 5S
4. MRX-3 ON ROAD CONVERSION 1512, MMM ON 6S
5. TEN T 2650 T8, MMP ON 3S