Have you wondered why the red apple looks red but not blue or green? All these have to do with the properties of the photoreceptors which are either rod or cone cells on the retina layer of our eyes.
Newton had previously observed that colour is not inherent in objects. Rather, the surface of an object reflects some colours and absorbs all the others. We perceive only the reflected colours.
Thus, red is not “in” an apple. The surface of the apple is reflecting the wavelengths we see as red and absorbing all the rest. An object appears white when it reflects all wavelengths and black when it absorbs them all. Colour vision relies on a brain perception mechanism that treats light with different wavelengths as different visual stimuli (e.g., colours). Usual colour insensitive photoreceptors (the rod cells in human eyes) only react to the presence or absence of light and do not distinguish between specific wavelengths. Cone cells on the other hand, functions best in bright light and helps distinguish other types of colour.
Each photoreceptor can absorb a rather broad range of wavelengths of light. To distinguish a specific colour, the brain compares and quantitatively analyses the data from all three types of cone cells (detecting primary colours—red, green, and blue).
And our brain does this remarkably successfully—some research indicates that we can distinguish colours that correspond to wavelength differences of just 1 nanometer.