The Color Spectrum

The source of color is the visible light. 
Visible light exist from a large number of colours (Electro magnetic radiation of a certain frequency). 
These colours are ranked in a solid order: The spectrum. 
It starts with a purple-blue color, fading into cyan, green, yellow, and eventually, red.
If you recognize this order of colours that can be correct, because it is the exact spectrum of a rainbow.

Absorbing

In general, light with its full spectrum, is all around us. 
But still we see everything in different colours. 
The reason for that is that objects (of all surfaces) absorb certain light frequencies, and reflect others. 
There are also objects, like paper sheets, that reflect almost all frequencies, or objects that reflect almost no color at all, like a hamburger that burned on a barbecue for too long. 
The properties of surfaces where light reflects on decide what light will be bounced back. 
This controls how we can see colours from objects.

Cones

Our eyes contain lots of light sensitive cells. 
The bars share the same pigment, so they are not suitable for perceiving different colours. 
What they can see is the contrasts between colours, the difference between strong lighten objects, or very dark objects, or anything in-between. 
The bars also work at dusk, something that can’t be said for the second group of light sensitive cells in our eyes: The cones. 
These do need light and can be grouped in 3 pieces: Cones with red, green, and blue pigment, sensible for red, green, and blue light. 
That’s why we can say that white light contains 1/3rd of red, 1/3rd of green, and 1/3rd of blue light.
Even if we know that the total spectrum exists out of multiple layers of light.