In 1861, James Clerk Maxwell was the first to create a color photograph through an additive process. After Thomas Young found evidence of the three different color receptors in the eye, Maxwell's method demonstrated how the colors red, green and blue could combine to create a complete photographic image.
The subject of this demonstration was a striped ribbon. Maxwell made three separate black-and-white images, each taken through a different colored filter: one red, one green, and one blue. From the black-and-white negatives, he made black-and-white positive transparencies. When the transparencies were projected onto each other, each through the same filter it was taken through, the complete color image was re-created.
Modern color photography is based on subtractive rather than additive processes, but early subtractive color methods used the same principles of separating colors through red, green and blue filters.
Extra notes:
Maxwell's color filters were glass containers of various chemical solutions.
Early photographic emulsions were not actually sensitive to red light. The red stripes in the ribbon still managed to show up, though, from the fact that the red dye also reflected ultraviolet light, which the emulsion was sensitive to. This was not intentional - it was discovered a hundred years later through a re-creation of the experiment by Ralph M. Evans.