[QUOTE=dr bob] I made the mistake of thinking a Wratten 11 or 15 would bring out the "sparkles" in a local scene - wrong! The result was completely dark shadows without any detail (TX400). I rephotographed with a Wratten 47 (blue) and the shadows popped out perfectly. This seems to fly in the face of general logic until one considers that the shadows in a sunlit scene are illuminated by blue light.
Dr. Bob it seems to me that your stated experience verifies that shadows are illuminated with blue light. Yellow filtration would be minus blue and that would account for the deepening of shadow values that you noted. The 47 blue filter is plus blue and would account for the lightening of shadow values that you indicated.
This blue filter would have the effect of lowering local contrast within the snow itself since local contrast within the snow itself would contain small shadow areas that are lit by the same blue light that you noted in the shadows.
While a full scale scene may have shadow and snow both included, the actual scene would need to be evaluated to determine the exposure and development considerations. However for local contrast in the snow itself yellow filtration and expanded development would enhance local contrast.