Thanks for the suggestions.
I tried the lighting approach, this works but RGB(255,255,255) would show the image as normal, and only be able to make it darker. This wasn't exactly how I wanted it. Also the .png seems a good approach, but I couldn't get it to work well as I wanted (it was changing the underlying images in ways that looked too much like an overlay).
I think for now I will stop trying to do this, but in another project I may try it. I think if you start with images that are full daylight maybe a bit too bright, you can do this with a simple light, like mentalthink suggested.