The problem with the trichromatic theory of color vision is that it did not adequately explain why staring at the reversed American flag produced an afterimage of a flag with the correct colors.

The trichromatic theory of color vision only explains how the eye detects and processes the three primary colors (red, green, and blue). It does not account for color afterimages or the perception of color combinations that are not directly related to the three primaries. The afterimage of the reversed American flag is an example of a color afterimage that cannot be explained by the trichromatic theory alone.