A second problem is the failure to describe the important effects of strong light. The appearance of colours reflected from a surface (such as paints or inks) is not the same as mixed colours of light. "Colours" such as browns or ochres cannot appear in mixtures of light. A strong lightness contrast between a mid-valued yellow paint and a surrounding bright white makes the yellow appear to be green or brown, while a strong brightness contrast between a rainbow and the surrounding sky makes the yellow in a rainbow appear to be a fainter yellow, or white.