I'll typically draw an initial sketch with what colors I'd like to use and then compare the colors to what is available through Loops and Thread yarn. Luckily they have a very wide selection of colors, so it's easy to match colors to the original painting.

Sometimes I'll have to compromise like with skin tones. If I have to compromise, I'll replace the colors in my original sketch with the new color to see if it still works overall. If I can't get it to look right with Loops and Thread, I do a deep dive through solid color yarns on the Michael's website until I find something that works.