Some are on white shirts, some are on black. It is best to design everything for black shirts so if they decide to do white, you still have the black shirt design ready to go. To design just for white and then have someone tell you they want them to be on black later on down the line is a p.i.t.a. I can't really describe to you how to make them turn out the way they do. It is all about the color layering, white base, the order in which you print the colors, etc. etc.
Here are some examples on black...
These designs were only intended to go on black and are supposed to be dark and grungy looking. No bright outstanding colors in here...
But this one here was intendde to go on both white and black and the design needed to stay true for both, so it was done a little differently. It looks just as good on either shirt.
These here were only intended to go on white. That was a good thing because it just wouldn't look as good on any other color.