I was reading this blog post: http://ianstormtaylor.com/design-tip-never-use-black/
Which phenomenon explains this:
But I must have been thinking the same thing, because one of those days in art class Mrs. Zamula came in with a blue light bulb to prove it. She screwed the bulb into a clamp-light, plugged the light in and clamped it to a stool. Then she got a pure-white ball from her cabinet full of pure-white things (that she kept for figure drawing exercises) and placed it on a pure-white pedestal, under the light. And sure enough, when she turned on the blue light, the shadow cast by the ball was an orange tint, not black.