The set of all valid RGB colors is a cube: [0,255] x [0,255] x [0,255]. The first two PCs is a planar slice through that RGB space. If you take a cube and slice it with a plane, you will get either a triangle, quadrilateral, pentagon, or hexagon. The graph of the first two PC scores is a projection from the cube onto the PC plane. The probability of a diamond is smaller, so you don't see it as often.