What is the direct physical cause of a negative afterimage?

Answer

Photoreceptors for the original color become temporarily exhausted

Staring at a bright color fatigues specific photoreceptors. When shifting gaze to a neutral background, these fatigued cells under-fire their signals, causing the opposing, un-fatigued cells to dominate the resulting perception.

What is the direct physical cause of a negative afterimage?
causevisionopticsillusionperception