"Impossible colors" or "forbidden colors" are perceived colors that only exist through optical illusion. Common examples include self-luminous red (a pink that appears brighter than white), stygian blue (a blue that appears darker than black), hyperbolic orange (an orange that appears more than 100% saturated), yellowish-blue (a true mixture of yellow and blue that isn't green), and reddish-green (a true mixture of red and green that isn't yellow or brown).

More information, along with visual examples, can be found here: "Impossible color" on Wikipedia