To understand this topic, you must firstly understand how we see colours. We all have these parts in our eyes called cones, and when cones get activated, we see colour (according to which cone was activated). Cones aren’t activated by colour though, they’re activated by wavelengths/frequencies, as each colour has a certain wavelength/frequency.
Some colours have long wavelengths (red, orange), others have short (blue, violet) wavelengths, and some have medium wavelengths, (yellow, green). To see colour, our cones first sense wavelength, after that the according to the wavelength, a cone gets excited. Finally the brain sees the cone got activated, so it shows us the colour.
Our eyes only can only comprehend wavelengths of 400-700 nm according to N.A.S.A, a company that has revolutionised the world with sending man to the moon and more. Since there are still wavelengths ranging from more than 700 nm, to less than 400 nm, we know for sure that there are more colour out there.
These colours are ultraviolet and infrared. Colours with wavelengths less than violet are called ultraviolet, meaning a violet to violet we can’t imagine it. Colours with wavelengths longer than red are called infrared, meaning a red so red we can’t imagine it. Here is an image from that shows you the colour/light spectrum.
If we can’t see infrared or ultraviolet, how do we know it’s real? Well, we can’t see them, but we can feel them. When you go near an electric stove or a campfire, you feel a heat (without touching the fire). This heat is in fact infrared light.
Ultraviolet is also heat related, as ultraviolet light is the light that gives you sunburns. Our body is extremely sensitive to ultraviolet light, that is why it gives us sunburns. This is all proved by an experiment conducted by Samuel Pierpoint Langley, he got a prism and let light flow through it, the spot the prism was directed at, became hot, due to ultraviolet light.
You might be thinking why should we care for colours that we can’t see. It’s not like anyone can see them. But that is where you’re wrong, certain beings can see them. Animals can have different amounts of cones, so some animals do see more. For example dogs only have 2 cones (green+blue), so they see less than humans, they may not be able to see the fiery reds, and bubbly oranges we see.