If by creamy you mean "smooth",.. a lot of times this is done by airbrush, so as to not leave any brush marks behind in the paint. Also, some of it could be a trick of the eye. Subtle shading, by using a coat that's ~almost~ the exact same color, just a hair lighter,.. and then another coat just a hair lighter then that,.. and then another, and then another. If done right, it all appears to be the same color to your eye, but gives you depth. And then the opposite is done in the areas of shadow,.. a slightly darker shade,. then another,.. then another. The contrast is what makes you see the "richness" in the work.