Everybody knows these free psychedelic color effects. A little gentle pressure on the eyeball is all it takes. But what causes them? What kind of neural firing happens to make these light shows?
I make 'em by closing my eyes and kind banging my eyelids (not too hard!) with the tips of my fingers! 
You can also generate phosphemes by directly stimulating the optic nerve with an electrical current of a few volts. A wave form generator or muscle stimulator that can produce square waves from 1-50 Hz works pretty well for this. Simply hook the electrodes up to the right locations around your eyeballs, turn a few knobs and enjoy the show !
If you want to experiment with a phospheme generator, Scientific American ran an article describing phosphemes and phospheme generators in the late 60’s or early 70’s. It shouldn’t be too hard to find at the local library.