I painted a motherboard 3-4 years ago. It worked perfectly fine. Just mask off the bigger ICs on the board so they can expell heat. It would probably be a good idea to mask off all the connectors you need as well :p Also make sure you buy a non conductive paint. To test, spray some of the paint on a flat piece of metal and wait till it dries. Then use a multimeter to test resistance. If 0, you'll be good.]

Don't use a gloss paint, it really won't look good.