Doing one of my Grandaugter. Will do another Marylin if it works out also.
This is how I'm doing portraits. I haven't gone all the way thru with any this way yet, but so far I think this is going to work out well.
I use cutout filter in photoshop to get this effect. I number the values. I will number the values accordingly on my palette with real paint.
Paint these in and softly blend them. I get as close as i can with the values. But I anticipate small adjusting later on. Feel a bit like I'm painting by numbers, but who cares if i spent 10 years or 10 days on a painting. It's the outcome.