hate to be a spoilsport, but only the dark squares in no shadow and light ones in FULL SHADOW are 120/120/120. The light ones are otherwise 206/206/206. The dark ones in shadow are 62/62/62. Also a thicker 120/120/120 bar only matches the dark squares as expected. The illusion ONLY applies to squares A & B and other light squares in full shadow, ie it amounts to a visual underestimation of the degree of shade. A selection 120/120/120 copied to the edge of shade sticks out.