You can try painting heatsinks but taking them off from motherboard might invalidate the warranty and also require replacing thermal tapes used to connect them with components they were cooling.
When picking a paint you need to make sure its right type for material that heatsinks are made of (especially if they are made out of aluminum) to ensure there wont be any reaction going on.
You need to either scrap all the old pain from them or again, make sure that new paint wont go into reaction with old one.
If your heatsinks are not painted but anodised you need to either scrap it or put a paint layer on it to prepare them.
Then your new paint needs to be heat resistant, you dont want it to start emitting chemicals when radiators heat up...