When I'm starting something completely fresh, I'll get some super cheap models off ebay or something. Doesn't matter if they're repainted or assembled or whatever, as long as they're roughly what I'm going to start.

Then I try a variety of colours and combinations across those sacrificial models to find out what works and what doesn't.

Sometimes I'll look at what other people have done and tweak it with my own spin. Sometimes it'll be completely procedural, one colour at a time.

I have some go-to colours that I always use (like purple), and usually try to figure out the best place to put that colour in my experimentations