Only the red elements in the photographed scene reach the negative back of the red filter. So also the negative back of the green filter registers only the green and the negative back of the blue filter only the blue. Suppose we are taking a picture of a red barn on green grass against a blue sky. Since the barn reflects only red light, the barn must register itself on the red negative. In the same way, the green grass sends its light to the green negative and the blue sky leaves its impression on the blue negative. But although it is convenient to speak of the red the green, and the blue negatives, it must be remembered that these negatives are not actually colored. The red negative, for instance, has no capacity for turning red when the light strikes it. It gives merely a black and white record of the red element; it is an orthodox photograph of the red portions in the camera's field.
The red, green, and blue negatives do however embody the intensity of the light that strikes them. And the next step is to preserve these values in a special positive that will absorb and print the dyes. This positive is a special gelatin-coated film. Light is sent through the negative to the positive. Then, through a special chemical process, the gelatin of the positive is hardened in proportion to the light that strikes it.
For illustrative purposes let us follow the process from here on the red negative. Wherever the object photographed is reddest, the hardening of the gelatin is thinnest. Because, of course, the red light blackened the negative and now keeps the light from the gelatin. When the red positive is taken out and washed, the soft gelatin is washed away. And the remaining hard gelatin forms a relief of the red record. In this relief, the red portions of the scene are represented by valleys and the blue and green portions are represented by hills. The positive, with the hardened gelatin, is called a matrix. Remember that on this red matrix the high spots occur where there was the least red in the photographed scene and the low spots where there was the most red.
The next step is to dye the matrices. The red matrix is brought into contact with a blue-green dye called cyan. This blue-green dye is the opposite (or complementary) color to red. The green matrix is dyed with a magenta dye which is complementary to green and the blue matrix with a yellow dye which is complementary to blue. So each matrix is dyed with its opposite color. But when the red matrix is brought into contact with the blue-green dye, the dye is absorbed only in proportion to the thickness of the matrix Thus the hills on the matrix, which act like type in printing, get a great deal of dye and the valleys get little or none. Since the red barn is a valley on the red matrix, the spot where the barn registered does not get any blue-green color. But the low place on the red matrix must necessarily be a high place on the other two. So on the blue matrix and the green matrix the places where the barn did not register must be hills and must have received plenty of the yellow and the magenta dyes.
The final step is to print the dyed matrices on the final positive film - print in the printer's sense of the word, not photographer's. First the red matrix which, you remember, has been dyed blue-green, is pressed against the positive film. But, as we have just seen, the red matrix has no color in the place representing the red barn, so as far as the red matrix is concerned this place remains uncolored on the positive film.