Any idea what causes that type of coloration?

Protein spots

He's eatting good

Cool, I didn't know that's what caused the black spots. Thanks!

hyperpigmented melanosis, which is a fancy term for pigment discoloration. The condition seems to occur in larger adult fish more often than juveniles. The spots don't appear to be permanent. Often, the coloration often returns to normal after a period of time. And, researchers have found the fish exhibiting the spots seem to be otherwise very healthy.

They tend to get the Beauty Spots this time of the year and then it diminishes as the water warms up in May. Makes for some very pretty fish.