Loose and free oil painting
The look is of a study, or the initial blocking in before detailed rendering, or just the work of a painter, like Sargent, who generally eschews fine details.
Colors lean toward naturalistic earth tones, but you can push it toward more vibrant tones in your prompt.
No particular keywords are generally needed, but on the occasion it leans photographic, pull it back in line with 'painting sketch' or similar phrase.
Good enough to call it a v1.0 - but not quite as flexible as I want it, so you may see updates to this.