Quote Originally Posted by Bill Burk View Post
Not that you used a blue filter, but that the distribution of spectral energy on the towel might be predominately blue because you are in a shadow, taking light mostly from the blue sky. I only mention blue-filter exposure to support my idea.
Wow Bill. Now I understand what you are saying and it is a very plausible explanation. Thanks for your insight.

This begs my next question, why aren't (some ?) other people concerned with compensating for the colour temperature of the either the incident light or the colour of the reflected light when exposing their B&W films ? In my accidental discovery I was able to show the equivalent of one non trivial N step in contrast (for want of a better term) between two different colour combinations without trying to find the largest possible deviation/worst case.