While the electromagnetic spectrum covers wavelengths between 100 kilometers and 0.01 nanometers translating to the longest radio waves to the shortest gamma rays, only a very small portion between 400 and 700 nanometers is visible light.
Visible light, as we are aware, is considered as white light in a band 200 nanometers wide and when we ‘split’ this band - say using a prism - we perceive colours - meaning, light receptors within the eye transmit messages to the brain, which produces the familiar sensations of colour.
At this point, we need to remember that colour is only an ILLUSION. There are no colours in light.
400 nm wave appears VIOLET to most of us*
445 nm Indigo
475 nm Blue
510 nm Green
570 nm Yellow
590 nm Orange
650 nm Red
* All of us do not see the same colours!
So when we see light, we see the entire band of 400-700 nm and it appears white to us, but it is little more complicated than that.
In fact, we cannot 'see' light at all. And it is not "white light" really. It is just ‘light’. Only when light 'reflects' off objects, we see the objects and not the light itself. Most objects around us absorb light too. So, when light falls on a leaf of a tree, for instance, the leaf absorbs all the wavelengths except 510 nm which it reflects, and we perceive GREEN colour. When light falls on a flower, it may absorb all the wavelengths except 650 nm which it may reflect, and we see RED. (no pun intended).