I have let parts sit overnight in the dye bath and finished them the following day. Rit dye comes in many different colors, I have only used black so far. If the process works well with black, the other colors should be a piece of cake. I have been told that a true black color is the hardest to obtain. If your part has a slight purple tint that is not dark black, it could have been caused by the part not being anodized completely, too short of a dying time, or the alloy type of the aluminum.
Cast aluminum is difficult to anodize and dye.
Rit dye is not light fast and may fade in direct sunlight. If this is a problem, use a professional dye instead of Rit. Rit dye also deposits a thick black powder residue on the part after it is dyed. The professional dyes hardly leave much residue at all. To remove the residue and restore the shine, use rubbing compound or wax for the final cleaning.