How does the film on a camera work to make a color picture?

Color film contains several layers of dye (substrates), each which is sensitive to specific color ranges of light. Since color OS made up from three basic colors, red, blue and green, the developing chemicals contain mixtures specific for each dye.

The amount of light within the color range as seen by the film will determine by how much it's exposed and the combination of all three will provide you with the different hues of combined colors.