Ewald Hering's Color Observations and Afterimages

Ewald Hering observed that some colors cannot exist together (e.g., reddish-green), while others can (e.g., greenish-blue).
Trichromatic theory was inadequate in explaining afterimages.
Afterimages are sensations that persist after a stimulus is removed.
Color afterimages involve specific pairs: looking at red produces a green afterimage; looking at yellow produces a blue afterimage.
Trichromatic theory cannot explain these specific color afterimage associations.