Quote Originally Posted by Bill Burk View Post
First thing that comes to mind is... Color separation negatives through the blue filter have to be developed longer than the other separations because blue light results in lower gamma. (That's an explanation I've read, others may corroborate or dispute). Maybe the light on the white towel had more blue than the cream wall.
Bill, I just found this effect you thought of reported here by Stephen and is called the Gamma-Lamba effect . The magnitude of my results correlate closely to those reported by Kodak for Tri-X.

In fact UV photography suffers from this effect and when I photographed the white towel it would have had optical brighteners from my washing liquid (that fluoresce UV) and possibly further amplified the effect

What a cool serendipitous discovery I stumbled upon in my testing.