According to Kandinsky certain colors (above) have an affinity for certain forms. A dull shape like a circle deserves a dull color like blue. A shape with intermediate interest like a square deserves an intermediate color like red. A dynamic, interesting shape like a triangle deserves an enegetic, luminous, psychotic color like yellow.
A hexagon is midway in interest between a square and a triangle so it gets the midway color it deserves, orange. Toilet cover seats get green.
Lines also have an affinity for certain colors. Bold, dynamic lines like diagonals get a bold color like yellow. Less drastic diagonals get a less drastic color, red. Dead lines that are nearly horizontal get a dead color like black. Slightly active lines like verticals get a dull color like blue.
Kandinsky even has a theory about coloring lines according to their centrality in the composition. Lines in the middle get yellow. Sad, unloved lines that hug the edge of the frame should get dull colors.