E is "street lingo" for Ecstasy, or XTC, some people say X. So is skittles, m&m's, smarties or any colored candy band you can think of. As E (as in xtc) can come in many colors, also with stamps/logos. Uh, without going into too much detail, logos is the brand and colors is what the MDMA is mixed with (if anything). So, usually someone would go for a, uh, purple playboy or a blue buddha, etc, etc. It varies from place to place, and the source, etc...
That really needed to be followed by: Not that I would know personally or anything...
At the clubs I work at in SF we just call them E-tards.