ELI5. Why mixing colours give us other colours. Like what’s the science behind?


ELI5. Why mixing colours give us other colours. Like what’s the science behind?

In: Chemistry

It’s just how our eyesight works. We have three types cells in our eyes which are triggered by red, blue or green light correspondingly. Different combinations of these basic colors produce perceptions of different other colors. Other animals (like birds) have four types of color receptors in their eyes, so they are much better at discerning various color combinations. And in fact some rare humans (mostly women) are also tetrachromatic, giving them better color vision.

That’s how our eyes work. We have three types of cone cells in our eyes, “red”, “green” and “blue”. Each of them [reacts to different wavelengths](https://www.unm.edu/~toolson/human_cone_action_spectra.gif) of light differently: they produce a strong signal when they “see” light corresponding to their colour but the signal gets weaker if the wavelength differs from the peak point.

The combination of these three responses is what we call a colour. But we can combine light in different ways to make our cells perceive it as something else: for example, a mix of red and green light will be seen as yellow since it produces a stong reaction from red and green cells but weak reaction from blue cells, just like “normal” yellow light does. That’s how screens work.

Mixing paint works in a kinda similar but inverted way. Paint doesn’t produce a coloured light, it absorbs part of light falling on it. White light contains the entire spectre and, for example, a yellow paint can absorb parts of it that are far away from the yellow colour leaving only yellow part to be reflected in your eyes. When you mix paints, you take away different parts of the spectre, leaving a combined colour.

Visible light is made up of a mixture of different wavelengths (colors). When hitting something all these are absorbed by the object except for the 1 (or a few) which is reflected. This reflected light is the color we see. If you look up to a light spectrum you can notice that these wavelengths gradually change the color, permitting us to see the difference between light and dark orange for example (as a specific wavelength will appear for dark orange and one for light orange). Some wavelength not present in the solar light spectrum can be seen as a fusion between 2 or more reflected wavelengths.

When mixing colors we are not doing anything but adding more waves that will be absorbed or reflected, ultimately summing to the ones we already have and changing the color we see. mixing blue with yellow will make make the compound reflect both blue and yellow waves, which combination will appear green to us (the high frequency yellow wave summed with a low frequency blue wave will give and intermediate frequency green wave).

White surfaces reflect all visible light (and hence reflect the combination of all these wavelengths together, which it appears white to us) while black surfaces, being the sum of all colors, will absorb all the colors and reflect nothing back (so we wont be able to see that object and it will appear black, like the absence of light).

Different elements and compounds absorb light particles of different wavelengths and reflect others. Something “Red” absorbs the other colors and reflects the red light back out. When you mix, say, paints, it changes what is absorbed or reflected to your eye – part of it is reflecting yellow, and another red, so you see red and yellow, but the bits that do the reflecting are mixed up so the light comes together as orange, say. Eventually if you mix enough pigments correctly, the combination just absorbs everything and you get black.