Large grain, preformed bits of pills, so that mixed together, they'd look like 'hundreds and thousands', them pressed into pills so that each pill was different (but the same, if you know what I mean) - a mottled pill made up from lots of different bright colours (or even flourescent, if there are fluorescent food dyes - have them glow under UV light).
All of the different coloured bits would have to contain MDMA at the same concentration, to ensure the pills all contained the same dose. Sort of like a multicoloured marbled effect.
For a dye stamp though, I've seen diazepam tablets (in spam for online pharmacies) that have a heart shaped hole in the middle. Do that with MDMA and it about says it all!
What the hell, multicoloured marbled pills with a heart shaped hole in the middle (that glow in the dark!)