You don't need thicker paint, you need a different undercoat.
Under coat flesh with white as a base. You can apply yellows, reds, and browns to the white for different effects in different areas, too (essentially pre-shading.)
The pigment loads of many pastel colors are relatively low. Even with the addition of hiding agents (essentially colorless semi-opaque pigments) their hide is poor. Undercoating with white greatly increases surface reflection, and will actually make the skin tones more vibrant.