Have you ever bought a bottle of polish that looks like its a creme but, when you apply it is too sheer and not the look you were going for? Well, I like to use Orly Ridge Filler Base coat (its white) as my base coat for these kinds of polishes so they will get that creme look like the way the polish looks in the bottle!
I've also read that you could use a white polish under those kinds of sheer colors too.
Examples of colors that would be best to use these methods with: OPI Give Me the Moon, OPI Need Sunglasses, OPI Make Love. Just to name a few.