How can I do angle-based colors (like the Pink Floyd "Dark Side of the Moon" album)

[Ofnuts]

First solution: do a rectangle, fill it with rectangular strips, and then distort the rectangle with the perspective transform. You can use regularly spaced guides to help you make the strips.

Second solution: use guides to position the start and end points of the boundaries, then use the guides to make trapeze selections that you bucket fill. This is a bit more complex than the solution above, but you should get slightly sharper edges between the colors.