Quote Originally Posted by PeterB View Post
A hypothetical example - the reflectivity of indigo is much less than yellow. If I had two scenes , one with indigo in a shadow, the other with yellow in shadow (same incident light level to both colours) and I wanted to place them both in zone III, they will both turn out to be the same shade of grey. That isn't good since I'd like the indigo to be darker than the yellow.
Peter, I know what you are talking about, but you've stated it incorrectly. If the reflectance is different for two objects under the same illuminance, they will have different luminances; and barring any spectral sensitivity problems, the resulting exposure will produce different densities. What you're talking about is producing different tones from two objects of different colors having the same luminance.

As Bill said, color filters are the answer. I've attached the Maxwell Triangle which "can be used to predict the effect of color filters on black and white film." As the color of the filter moves further from the subject's color, the resulting tone progressively darkens. Complementary colors darken. Supplementary colors lighten.