The short answer is that it's your uke and you can do anything you want to it!
The slightly longer answer is that if you're talking about painting the "corners" where the top and back meet the sides I wouldn't really bother. Traditionally, binding (thin strips of plastic or wood) is used there on some instruments to both hide the seam where the two pieces meet and to provide protection. Some very cheap ukes from days gone by used painted "bindings" that basically just tended to chip and look ugly without providing any protection. There are some very fine ukuleles made that have no binding - my KoAloha has no binding and it cost several times what many ukuleles that are bound cost.
But, it's your uke, and if you want to paint fake bindings, or unicorns dancing across the top, that's nobody's business but your own!
John