I'd be very careful, as paint, plastic, and stuff you touch a lot with your fingers does not mesh well in my experience.
Even the plastic gets worn down on my keys, and paint-y gummy fingers would be unpleasant.
Might want to look into some vinyl dyes for the keys, or just paint the keyboard frame which gets touched much less than the keys. Some other alternatives here:
If it is a keyboard you plan to use heavily, painting the keys is .. risky. I'd start with the frame first.