Usually one frame has a colour chart shot under exactly the same light conditions, or another sheet of film. The lab, or whoever is doing the printing will correct colour and density for the frame/sheet with the colour chart, then print the other frames/sheets exactly the same.

Sometimes we used to be asked to not even change density, meaning if the neg is thin or dense, then the print will reflect the under or over exposure the photographer intended.

One used to see some quite interesting things, especially with regard to fashion shoots. The idea being that the wackiest frame was often chosen, then that C41 frame/sheet was copied to colour print film, which was a C41 process that was exposed under an enlarger but developed in C41 developer, with the end result being a transparency. Mostly these transparencies were used for four colour printing on printing presses, or magazine reproduction.

Mick.