The color of film exposed and processed in a laboratory is controlled by separately altering the amount of time that the red, blue and green lights are used to expose the film. This is referred to as color timing and its effect is to alter the contrast of R,G and B to create a required color balance.
In a lab, color timing is usually applied at the point where the edited negative is copied to the master interpositive, but can be done later at other points, if required. This contrasts with the digital intermediate process where color timing is applied at any required time. In addition there is far more flexibility for color control with gamma, hue, luminance, saturation as well as secondary color correction. Also the results can be seen immediately and projected onto a large cinema screen, and further adjusted if required. The images have precise color settings to show the results as if output via film, or digitally.