Making diagonal lines on a set of pixels

I have a province class, which has a list of Vector2s called pixels. I have a script that takes each of these pixels and colors them to the color of the province's owner. My game is a grand strategy game, in which people play as countries, and an important part of the game is war. In wars players occupy enemy land, and I need the color to change to reflect that. I could of course change the color to that of the occupant, but the lands ownership doesn't change in such an instance, just its occupant, so I was looking for a way to make diagonals across the land. So I want to change this to this as the occupant changes