iirc: A province is a single pixel of absolute white. That's all the game cares about on the map. So no other pixel may be exactly white (is it RGB 0,0,0?).
Borders and such on the map are optional, the game doesn't care about them at all.
the borders on a map are just for looks. a province on the map is determined by a single pure white pixel. when you click on a map in the game, the closest white pixel is found and that is the province that you get. nothing else in the image has any meaning as far as the game is concerned.