I was told in art lessons back in the Cretaceous that neither black nor white were colours. So I guess from an artists point of view this supports what many of you are saying.

Mike's comment, "B must have a tad more Black in it than A, due to the shadow cast across it by the green pole?" is basically correct. You put black into a colour to give the appearance of shadow. The shadow on the light square has made it as dark as the light on the dark square. (I'm of Irish ancestry if you have trouble following that.) Of course purist artists wouldn't use black pigment. They'd mix a brown (the three primary pigments mixed) and add a dark blue. This comes out a more interesting black. I often used to paint using just three primary colours and white.