These are the scans of watercolour swatches and sketches that I've created in my Youtube video.
You can get nice skin tones but the proportion can be difficult to get. You can't add French Ultramarine to it because the granulation will make the skin look textured, unless that's the look you're going for.
These are the possible grays.
As you can see from the colour wheel, you can achieve rather vibrant orange and purple/violets but when it come to greens, those greens are really muted and dull. That's where adding a 4th colour, a green would make this limited palette more versatile. But which green? I prefer to use a green that can be mixed with existing colours from this palette. So in this case, I probably would add a Sap Green because you can get a colour similar to Sap Green by mixing French Ultramarine with Hansa Yellow. French Ultramarine is already in this limited palette. I feel that it's easier to achieve colour harmony this way.