I think ChatGPT will just be calling out to a diffusion workflow to generate the new image, so this is using a general tool for a precise problem. It's not going to be great output.

I think, given you know some details about the film you're scanning and how it reacts to certain hues/brightnesses, you can remap colours from greyscale to a few possible colours right? Obviously it's not a 1:1 mapping since there's some data lost. (A dark blue is going to be a similar grey to a brown or something) but I'm almost certain there's old ML technology that essentially takes the most likely colour based on the context of the entire scene/some convolution matrix chunk of the scene, trained for specific film formulations.