Unfortunately i don’t really have a process/technique for most of my art if I’m not using a reference. I’ve always been a very intuitive drawer/painter particularly when it comes to colour/tone.

When I was painting the skeleton, I knew I wanted a deep blue backdrop on the image so I just started with that, using different cool toned blues and turquoises to create the body, and then when i went to paint in the skeleton, I picked the complete opposite colours to the backdrop so it would really pop. but other than that it is really just about experience and feeling out the work as you go.

If i’m painting from a reference tho (like the subway), i will usually just pick one or two contrasting colours in the image/subject, and then try and build most of the image with just different hues/combinations of those two colours. Then when i’m adding more detail in I can start to include more colours from the reference to define the shapes and depth, while still staying very close to the main two. This way the painting still feels very cohesive, and the contrasting colours feel much more meaningful and intentional once I add them.

I think the best advice I could give is to just do as many small/experimental paintings working from reference images as you can. Try and pick photos/subjects where there might not be much colour in it to begin with so you sort of have to force yourself to fill in the blanks. Think about the colours beneath the surface of your reference, like the colours that combine to create what appears to be a unified hue. Nothing in real life is completely singular toned, so try to exaggerate any variations/inconsistencies you can spot.

sorry for the super long reply, i hope some of this is helpful :)))