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Vincent, welcome to the forum. One thing Mark always says is that skin tones are just like any other colors. If you are working from a photo, laminate it, dab the paint you mix up right onto the photo, wipe it off, mix a bit and repeat until the color matches exactly. Then place that mixed color in the same spot on your painting that you just matched on your photo. Almost all of Mark's videos are free but there is one where he paints a portrait while explaining his processes. That video is a few hours of fantastic. It costs a paltry $20 and has taken many painters up a notch in the quality of their paintings. I recommend it just like all of his other videos.