Some People Have Four Primary Colours

Most humans have 3 primary colours. The eye has photoreceptors called rods which measure light level and movement and cones which measure colour. Our cones are of one of three different types, each seeing a different wavelength and these roughly correspond to Red, Blue and Green.

The three peaks of colour reception for humans.

When the brain receives inputs it has to map it onto something so that you can comprehend the data in a useful way. With the concept of colour the brain made a weird decision and decided to map something which was very one dimensional, wavelength of light, and map it onto something two dimensional, the colour wheel. By cutting out anything ultraviolet and below, or infrared and above, then looping the red end of visible light onto the violet end we get a spectrum of colours which seems to go around and around. To our brains it looks like we can make any colour out of some combination of Red, Green and Blue.

But what about Yellow as a primary colour? Well it is actually arbitrary, all that matters is that we have 3 primary colours, because that is all we need to make up the rest. If you have seen Cyan, Magenta and Yellow as the primary colours of printers you will have seen this in action.
Not all animals have 3 cones, for instance dogs don’t have a cone near Red and only have two in total. All colours are some ratio of blue and green to them, purely 1D. If you are choosing a dog toy then you should avoid the colour red: dogs have trouble finding a red ball in the green grass.

Other animals have more cones, so Reindeer have 4, Butterflies have 5 and Mantis Shrimp have 16. Remember that this isn’t how many colours they can see, it is how many dimensions they can see their colour in (ish). A mantis shrimp looking at a colour will see it as some unique combination of its 16 primary colours. It is simply unimaginable to us stuck down on three.
There are lots of theories on why the mantis shrimp has evolved such vision, but the main idea seems to relate to distinguishing between the patterns of the bodies of the males for sexual selection.

Some humans are born with 4 types of cones rather than 3 and the name for them is tetrachromats. Females tend to be better at seeing colour, with far fewer colour-blind individuals and these is also true with a average cone number: almost all tetrachromats are female. These individuals see colour as a combination of 4 primary colours!

Usually humans can’t see UV because the lens filters it out of the light hitting the retina. However there are some humans born without lenses and they can see into the UV spectrum. It is commonly reported as either whitish blue or whitish violet. Many things like flowers have extra patterns in UV because the insects that pollinate them can see into this range. Interestingly most insects can’t see into the red end of the spectrum and so all red flowers are either human bred or have birds as their primary pollination method.