the development of dark colors on parts usually exposed to the sun and of light colors on parts usually shaded, especially as serving for protection or concealment.
Origin of countershading
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
(in the coloration of certain animals) a pattern, serving as camouflage, in which dark colours occur on parts of the body exposed to the light and pale colours on parts in the shade
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012