I’m presently working on a portrait. I did the skin tones with Burnt Siena which makes a passable skin colour. Once I had the tonal values close I went back and added colour; blues reds and magentas, this looks much more realistic. Why? I think the colours are actually there but we don’t consciously notice them after they’ve gone through our brain’s interpretation. A single skin colour appears flat, but a plethora of them appears natural because our brain needs to do the same thing as it does when looking at a real person. I think this has a lot to do with reflection. Human skin is a lot more reflective than most people think. Some artists take this to extremes but it still looks right.