There are multiple ways to skin a cat, but the images you show could be reproduced by fogging the paper with appropriate light sources. Laser pointers come to mind. Keep in mind that the color you'll get will be the opposite in the color wheel from the light that's used for fogging. So yellow light will give blue fog, cyan light will give you red, etc. Light of very high intensity may fog all color layers and not just the color you're targeting; so e.g. in those final two images, a green light source may have been used and targeted at a small area which then became overexposed across all color 'layers', with the edges bleeding out into the complementary color of the light source used. Cut-out masks can be used to get the kind of shapes as shown in image #2.
Additional special effects can be obtained by chemical staining; e.g. small and very dilute drops of blix (let's say 1+20 or so) dropped onto the paper before it goes into the developer. You'll basically mess up your developer as well this way as it gets contaminated, so you'll have to resort to using it one-shot, or at least keep a separate batch of developer at hand for normal color printing.