Is it proper to state a Munsell notation for a metallic or pearlescent color, or is Munsell a solid color only notation system?
It can be done, but it should be done with great care. With metallic or pearlescent materials, the color changes with changes in illumination and view angles. Thus, if you wanted to use Munsell notations for such materials, they too would have to change with illumination and view angle and the material would have quite a large number of Munsell notations.
