The opponent process theory was created by Ewald Hering. This theory claims cone photoreceptors are paired to form three opposing color matches. These pairs include blue/yellow, red/green, & black/white. These pairs can only detect the presence of one color at a time because when one of these is activated, the other pair is inhibited. This explains why we do not see “bluish yellow” or “reddish green” because the opponent cells can only detect one of these colors at a time (D'Mello, n.d.). In addition, this theory explains  after-images. When you stare at an image for an extended period of time, when you look away, an after-image is often seen in complementary colors.  When you are staring at this image, your cells for that color fatigue. When you shift your attention, those color cells cannot fire so only the opposing colors continue firing, leaving a brief after-image in the opposing color (Loken, Voytilla, Bach, & Sirisanthana, n.d.).