A lot of people have custom-painted ukes, including Flea/Fluke soundboards. I have a painted Fluke and it sounds just the same as the regular one I had before. The paint on it is rather thick, too, so I'm surprised there's no loss of volume.
If you do a search, you'll come across some threads about this subject. If you want to paint a Fluke, I think it's a great idea! They are ideal for adding your own personal touch.