hmmm....I feel like experimenting on how to do this myself when i get a chance, but I would maybe try placing the different colored ink on different parts of the screen and using small seperate squeegees (for the different colors)....stiff cardboard would work for this...so that the colors don't all get mixed together..I have not tried this, but it is how I would do it were I going to attempt it.
I am supposed to do a run of shirts tonight..maybe I will give it a whirl and post my results

The first three you listed appear to be color blends, where you simply deposit 2 or more colors on your screen in stripes. After a few passes on some test stock, it mixes into a very even blend between them:

Boxfresh appears to be 2 inks, screened sideways, Yellow and Orange

The Clown appears to be 4 (maybe 5) inks, Purple (and maybe Blue on the WAY far left, almost out of view), Red, Yellow and Blue

The Harajuku Lovers Shirt looks like 3 inks, Green, Yellow and a Rhodamine/Magenta

Now, the last one I don't think is possible in the same way. What it appears to be is two images superimposed (the Girl over a Rorshack Ink Blot), which was then separated into a four color process (i.e. the halftone dots are doing the visual mixing, as opposed to the inks actually co-mingling).

I'd done a piece with a simliar look once, but it was process color fades over succesive layers of acetate. Since the process colors are transparent in nature, it allowed the layers to "blend" in the final, but that was certainly an oddball substrate (acetate) that defied the laws of the standard piece (opaque)... :)

When you design your work, also remember that you can fool the viewer in how you do something by thinking in layers. There's an old Skateboard deck from the 80's whose Printers start going nuts with the variations.

There are at least three stencils at work for the background elements, each one was done as a color blend, but since the left edge of every layer was almost the same blueish purpleish color, it really makes you think "How in the hell did they do that?"

i122.photobucket.com/albums/...gonz.jpg

There's a Purple to Pink, a Blue to Yellow, and a Blue to Orange at work here.

This link might help.

screenmonkey.info/wordpress/