The C-41 negative film process arose from movie film that was marketed just after World War II. Previously, processing color films was a far more arduous task.

To simplify, three dyes, cyan (blue-green), magenta (red-blue), and yellow are incorporated into the film during manufacture. The dyes in this film is incomplete. The three dyes are all missing the same ingredient. That ingredient is in the developer solution. The developer is rather common black & white formula plus the missing ingredient.

As the film is developing, metallic silver forms in the exposed regions. Dissolved oxygen in the waters of the developer rapidly oxidize the surface of this newly formed silver. The action is the catalyst that causes the dyes to unite with the missing ingredient. Now the dyes blossom and a color image forms.

The problem is, the cyan dye is not the correct color. The magenta dye is good but could be improved. The yellow dye is worthy. The industry never was able to find better cyan and magenta dye that would fill this bill, i.e. one missing ingredient that is needed to simplify the developing process. For example, Kodachrome required three separate developer baths.

How to bolster the poor cyan and magenta that must be used? The undeveloped cyan dye was tinted red. The undeveloped magenta dye was also tinted. Together they appear orange. As they developed the orange tint is lost, but not in the unexposed areas. Thus the orange “mask” markup is actually two positive image superimposed atop the three negative dye images. Thus the image is comprised of 5 dyes.This positive orange mask corrects the off frequency cyan and magenta dyes. It’s complicated to make this film but easy to develop. The finished product yield pleasing prints.

The key here is “prints”. The printing paper consist of three emulsion layers, one for the red exposure, one for the green exposure, and one for the blue exposure. The emulsion speeds are not equal. The paper speeds are adjusted so that the red emulsion needs less exposure. The green and blue emulsion will be over-exposed unless these light energies are mitigated. Magenta filters to mitigate the green light exposure and yellow filters to mitigate the blue exposing light are deployed during the printing exposure. The red exposure is controlled only by the lens aperture and the time of the exposure. The bottom line is, the color balance of the finished print is controlled by adjusting the amount of yellow and magenta filters. In this way the exposing light is custom adjusted for a good color balance.

Typically, the color balancing filters used will be heavy on the amount of yellow filters and moderate on the amount of magenta filters. If the resulting print is off-color, the ratio of yellow to magenta filters is altered and this color corrects the final print.