I was asked on Quora to answer why when we mix primary light colours together they add up to white and when we mix primary dye colours together they yield black. Well here’s the thing.
Our eyes are receptors. i.e. they are like radio receiving antennae that are tuned to pick up particular wavelengths in the electromagnetic spectrum, and we call those wavelengths light. The wavelengths of these radiations are roughly in the range 400 -700 nm (billionths of a metre). We have receptors in our eyes called ‘rods’ that simply sense brightness. If we had no other receptors we would see the world like a black, grey and white photograph. But fortunately we also have receptors that pick up others wavelengths and they are called cones. The three types of cones ere each sensitive to a range of wavelengths but each peak at a particular wavelength. I nicked this image from Wikipedia showing the response curves of each type of colour receptor. The x-axis is graduated in nm. (Billionths of a metre).
if we projected wavelengths of 445 nm, 535 nm & 575 nm onto a screen, the images formed would trigger all three sets of cones and we would perceive three coloured dots or shapes something like this.
But in addition to being able to sense the three primary colours, our brains interpret a mixture of primary colour stimulations as a new colour. So here’s what we would see if we project overlapping discs of the above wavelengths of light.
But when we look at an object, say a picture or anything else around us, we are not looking at something that transmits wavelengths of light from within itself. Object around us are seen by us because light from somewhere, e.g. the sun or a lamp etc., falls onto the object and bounces off that object (is reflected) into our eyes. But some of the wavelengths in the incident light, i.e the light falling on the object, are not reflected into our eyes because they are absorbed by the particular object. Which wavelengths will be absorbed and which reflected is complex and depends on the atomic structure of the object and the wavelength of the incident light.
So for instance, If a white light falls onto an object, we know from the images above that means that all visible wavelengths are falling onto the object. That object then absorbs some of the wavelengths and reflects others. If for instance, the object absorbed all the blue waves of the incident white light and reflected the red and green wavelengths, we would see the object as red/green which we know from the above appears to us as yellow. You can work out from the above what the absorption of other wavelengths would lead our eyes to perceive.
If the incident light was not full-bandwidth white light but was say just blue, the object it fell on would only be seen by us at all if it could reflect blue light. If it was a green object for instance and we shone blue light onto it, we would perceive it as colourless or black.
Of course we are talking total absorption or reflection here, but if an object absorbed for instance, 80% of the red part of the white light falling on it. 50% of the the blue and 20% of the green, it might be rendered thus to our eyes.
The possible combinations are infinite.