Creating, comparing, and storing histograms

	A grey-green colour that often finds itself on the walls of public institutions—e.g., hospitals, schools, government buildings—and, where appropriated, on sundry supplies and equipment.
	-- "Institutional green", Segen's Medical Dictionary (2012)

I hesitate to make sweeping statements about the ideal color of paint on a wall. It depends. I have found solace in many walls of many colors. My mother is a painter and I like paint in general.

But not all color is paint. Some color is dirt. Some color is concrete or marble; plywood or mahogany. Some color is the sky through big windows, the ocean, the golf course, or the swimming pool or jacuzzi. Some color is discarded plastics and beer bottles, baked food on the stove, or perished vermin. Some color is unknown. Maybe the paint camouflages the dirt.

A typical camera can capture at least 16.7 million (256 * 256 * 256) distinct colors. For any given image, we can count the number of pixels of each color...