hope this tutorial helps, anon!! a lot of this is a result of experimentation and seeing what looks best based on your picture. what i’m essentially doing is relying on the overall “mood” bg colour tie everything together cohesively by putting the local colours over it on the “soft light” filter. the other changes i do, such as the blue light on him and the flavour elements are all based around making it more cohesive, e.g. the glow from the pool is all over the background AND the subject, instead of just the pool, pulling them together, if that makes sense!
of course this also relies on having some sense of what a realistic version of your scene would look like - for this comic i looked up “pool scenes night” on google to gather references i could take the colours from, and looked up lighting references for the way light would hit a face if coming from the bottom, stuff like that.
the other example is from a commission i did:
in this case, the lighting situation has a lot more light, and isn’t as atypical as pool-at-night lighting. so it was easier to rely on my usual method of colour layering, i.e.
local colour on multiply (as opposed to soft light)
monochrome shading on normal
again, experiment experiment experiment! hope this helps!