First, you should get the color that you want to paint your frame in the chemicals shops with only $5 to $8 per can. Second, before painting, you should disassemble the lenses and the nose pads. Then tighten the screws up. Next, you can begin to paint your frame. Tie an iron wire at the folded part between the glasses frame and legs to facilitate the painting. After the first painting is completed, wait for five minutes and then continue the second painting. Finally, let the glasses dry for 4 hours, and then install the lenses and nose pads.
The answer above give you a good suggestion. i don't have experience of recoloring my sunglasses but it is worth a try if your sunglasses are cheap. If it is expensive, don't take the risk of ruining them.