When painting skintones, don't just focus on values, also focus on tonality. You also want a variation of colors, not just the same color in various shades. A skintone I often use is off-white -> yellow -> pink -> orange -> brown -> gray, from lightest to darkest. If you use just a lot of shades, your colors will look muddy and not as 3d as you'd like. Also, make good use of hard shadows as well as ambient light to suggest depth and overlapping forms, as well as rim-lighting to accentuate edges. Edges are very important to defining form. there is not alot of rim-lighting if you're painting realistically but I think as artists we can take a few liberties or set up lighting situations where there would be.