There is a small chance it could reduce signal strength, depending on the exact materials of the paint. Metallic paints have been known to cause signal drop off on equipment.
All of that said, I would approach this differently. I would just get a black plastic electrical box, cut the back off it, and put the SmartThings hub in that. Cheap and easy.
If you get one from a supply house, look at “project boxes” or junction boxes. These will usually be better made and easier to modify then storage boxes. And that way if you decide to move it to someplace where white would work better, you just take it out of the box. Since smartthings doesn’t have any indicator lights for everyday use, you can hide it completely.
The following is just an example from a good brand, the sizing isn’t quite wide enough on this one unless it’s okay for an inch or so of the hub to be sticking out of the back. You’ll be able to find bigger ones, I just didn’t spend much time looking. Also, it’s best to take the hub to a supply house and make sure the fit is right. A lot of these will have interior features which may change the inner dimensions, and that way you can also check for color and translucency.