Quote Originally Posted by PeterB View Post
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
On second thoughts the optical brighteners wouldn't have amplified this gamma-lambda effect I observed as it wouldn't have created proportionally more blue light. The blue cast would have come from the fact that the white towel was in shade and only exposed to blue sky while the cream wall (already 'warmer' than white) was exposed to more direct sunlight which has a warmer colour temperature (see ct chart here)