I guess it's mostly because people kind of assume when people paint their body it's body paint. >.> Whatever the case, again, never ever use any type of real paint on your body. That's incredibly toxic; even if it says non-toxic, it's toxic. When they say they're using a spray paint for Mdina, they probably mean on the body suit... either that, or they did someohow manage to put their body paint in a spray can and do it. But I really don't know how as my knowledge on the subject is limited.
As for body paint, it's more makeup than actual paint. Again, I honestly don't know much about it. Ben Nye, I hear, makes a really good body paint. If you want some really good advice on the subject, you might want to ask the people who do Homestuck troll cosplays.
Concerning a body suit, both colors have their downsides. A white bodysuit, the black may come out badly, but at the same time it's way harder to paint white on black than it is black on white. But again, people might have spray painted their body suits, putting stenciling down on appropriate places. The spray coverage probably handles paining on either color better than hand painting.