Anything that is likely to cause an imbalance of the fan, is not a good idea.
At best it will cause premature wear, at worst, vibration.
Unless you can guarantee an absolute even coating of paint, I should leave well alone.
All electrical components emit heat. As most work on some form of resistance, heat is generated albeit in sometimes minute forms. However, when these components are designed, the designer takes into account heat dissipation, and your proposed little drops of paint are not helping in that respect. As processors become more and more powerful, there is already a heat issue within some poorly designed cases, and most folk are doing their best to reduce the amount of heat within a case, so whilst you may "get away with it", its not something I would recommend.