First, I'm brand new to oil painting and Marks method for color checking. I stumbled onto a site where you can take any photo and turn it into paint by number project. So I tried it and was pleased that I could view the colors used for any area of a photo. I used a photo I'm currently painting, but having some difficulty with skin tones. And other colors too. When I searched for a tone used in a shadowed cheek area, I was astounded that it's actually a dull gray-lavender. No wonder I was having problems in that area! Also, one person has bright red hair. But all the color samples are brown, yellow, not an orange I'd expect to see. If anyone wants to play around with it, it's https://pbnify.com/
Select your photo and click on the areas you want to see what their color is. A red circle will tell you where you are, and is saved, or you can delete it. You'll figure it out. I didn't go any further, like turning my photo into a paint-by-number project, but I loved seeing the colors I selected, enlarged into samples.
I'm aware that computer colors are a horse of a different color, so to speak, but this is another little learning tool I may use from time to time to help suggest what's wrong with a tone I'm struggling with.