For about as long as I've been painting portraits, raw sienna has been my go to color for mixing skin tone. I quickly found that it was perfect for the skin of caucasian people and I've also found that I can use to mix the skin of people of other ethnicities, by including colors in it, like green and purple. The skin for all the portraits below was mixed using raw sienna .
This method can be used for both impressionistic and realistic painting techniques. For realistic portraits, I recommend using the transparent raw sienna in Liquitex's Heavy Body line. So far, unfortunately, I don't believe they have it in the Soft Body or the Basics lines yet. I recommend that and either the transparent mixing white from Liquitex or Zinc white from Amsterdam Standard series.
Don't limit yourself when it comes to what colors you'll put in a skin tone mixture. Don't think you're only going to use browns reds and yellows. Depending on the skin tone of the person your painting, you might end up using any color in your paint box. This doesn't include all the shadows and highlights you're gonna paint on the skin either.
See some mixing demos and here more tips in the video below.
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Amsterdam Standard Series
Transparent Raw Sienna In Liquitex Heavy Body
Painter of portraits and wildlife