How is the depth of curing and shade of the material related?

Optical modifiers affect light transmission through a composite. Thus, darker shades and greater opacities have a decreased depth of light-curing ability and require either an increased exposure time or a thinner layer when cured. Studies have shown that for optimal polymerization, resins with darker shades and opacifiers should be placed in thinner layers. Phillip’s 12th edition page 291