Just be cautious that these cams tend to run on the warm side, as is. Changing their color (especially if they're in direct sunlight) could cause them to run too hot. As a general rule with computerized electronics, the hotter they run, the shorter their lifespan tends to be. Even if it doesn't initially damage the camera, I would still be concerned about the camera having a shorter than average lifespan if it were painted a darker color and put in direct sunlight.
As a side note, assuming this is a relatively new camera, painting it would almost certainly void any warranty. Even a lot of the Chinese sellers offer a personal year or two warranty with their own companies when the Hikvision manufacturer's warranty doesn't apply, so I would keep in mind that you'd be voiding any potential warranty that you may have.
One idea that comes to mind (though it may not be any better) would be if there were some type of insulation sheet that could be wrapped around it (sealed from water, if it's out in the elements). The outside of the insulation sheet could be painted dark while in insulation inside the sheet should block that added heat from reaching the camera...On the other hand, having insulation wrapped around the camera could also block heat dissipation into the air from heat generated by the camera itself. I don't know whether or not the net result would be a camera running cooler or warmer in an insulation-wrapped verses dark painted exposed surface.
I would also echo the other responses here that having the camera obviously visible has it's good points. When it comes to thieves, the better to stop them from stealing before it happens rather than let them steal and try using video to catch them. The thing to remember is just because you get video of the thief doesn't mean it'll be clear enough to identify them or that they will actually be recognized and turned in by the public. There's a lot of criminals caught in the act by security cameras that never have been arrested for their crimes. It's better to stop them before it happens and leave trying to catch them by video as a last resort.