achromatic colour BrE
achromatic color AmE

You are talking about achromatic colors," George R- said.

"The perception of transparency with achromatic colors."

Three computational biologists' theory to explain how humans perceive achromatic colors:

Pure achromatic colors include black, white and all grays; near neutrals include browns, tans, pastels and darker colors.

Some examples of necessarily non-spectral colors are the achromatic colors (black, gray, and white) and colors such as pink, tan, and magenta.

With W.S. Stiles he also developed mathematical methods to calculate by various methodologies the number of possible metamers for given chromaticities, peaking at the achromatic colors.

Since achromatic colors have no hue, the hue code (h code) is left blank for achromatic colors (usually marked as a dash).