We have done one color, once. Jackie has a red MAX-11S. Looks cool, but the red color shows carbon, grease and oil staining easily. Lighter colors would show it even worse. It's tough to coordinate manufacturing a couple of uppers being a different color and then the color is not perfect for what the customer was expecting. And getting repeatable color is almost impossible. Best bet for getting a different colored upper is to send it out for Cerakoting.
This is the exact reason my anodizer has decided to stop offering colors and does Model T colors (black).
That said I'd totally buy a blue or green upper