By mixing the colors into the white, as it is an acrylic that dries to a semi-translucent state, (it's a zinc white, not titanium) you can give the portraits a layered color depth that mimics the saturation in a person's face.

painting portraits on the cheap. 1 quart of Zinnser PermaWhite *satin* bathroom paint. 1 tube of mars black watercolor paint. 1 tube of spectrum red watercolor paint. 1 tube of medium yellow ochre watercolor paint. You can make ANY color human being and all shades and tones with the above list.