….even when using on-camera flash, you can use gels (like the Honl gels I use) for effect. Like in this Halloween evening picture (from which only the horns are missing), for which I used a red gel on the on-camera flash:

But won’t it make the entire picture red?

Evidently not. Why not? Answer after the click:

The flash does not make the entire picture red, because my settings are designed to also allow ambient light to do work. That would be around 400 ISO, f/4.0, 1/30th second.