When a camera's shutter opens, light photons stream through the lens, fall on some of the crystals, and dislodge electrons from the halide ions. Some of the electrons glom onto nearby ions of silver, converting them to neutral atoms. The atoms then cluster together, forming a tiny black speck of metallic silver on each exposed crystal. Millions of such specks form the image. To make a positive print, you shine a light through the negative onto a paper covered with the same kind of film emulsion, and then develop that.
Color film is more complicated.
It's a stack of silver halide emulsions, each designed to record red, green, or blue. There's nothing you can do afterward to change that. You can use the information, or you can squander it. The dopant would have to be a negatively charged ion, like a halide, so it could substitute for a halide in the crystal lattice. To neutralize a hole it would have to be willing to give up an electron, but not too eager, or else it would start neutralizing silver ions and blackening film that was still in the box.