We always used normal food dye, you can get a bottle at any local store, for $2 you can get enough to make thousands of hits... for a REALLY cool effect you can use a neon highlighter - the gellies glow under a black light (you can tell ravers it's "extra psychedelic" and make twice as much..)
But yes, the color has NOTHING to do with anything other then the dye that whoever made the tabs used... if the green gel-tabs are "stronger" then the red ones, it's because whoever made the red ones PUT IN MORE LSD!
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PACK.MY.BEAK