The three colour process stems from the fact that the human eye can see all colours by using a mix of red, green and blue which stems from the fact that within the human eye we have two different types of cells,cone cells which see colour and work well in bright light and rod cells, which allow us to see in low light. This is how pixels in a monitor or television screen today show colour.
James Clerk Maxwell first suggested in 1855 in a paper about colour vision this theory of how we see colour. He discovered that with the right mix of each spectrum of light, you can create any hue that stimulate the eye in the same way as it does when it sees real colours.To emphasize that each type of cell by itself did not actually see color but was simply more or less stimulated, he drew an analogy to black-and-white photography: if three colorless photographs of the same scene were taken through red, green and blue filters, and transparencies (“slides”) made from them were projected through the same filters and superimposed on a screen, the result would be an image reproducing not only red, green and blue, but all of the colors in the original scene.
The first example of a photograph using Maxwell’s prescription was taken by Thomas Sutton in 1861, his method however was severely flawed and colour photography didnt properly come into the light until the 1890’s.