If warranty isn't important to you however there is rather a lot you can do if you are careful and patient. My favorite technique is dye, you can remove the heatsinks from the board, sand them down to bare aluminum/copper then immerse them in clothing dye to color them without effecting thermals too much. Obviously for copper you would need to pick a color that will either go with copper or something dark that will hide it. I would not recommend using paint on the board itself however as that can cause all manner of problems, the right type of paint can be applied to anything removable that has not electrical circuitry involved however, including heatsinks.