My g/f had a white one.... they're a locally made speedbomb. They come from a source which also pumps out horseshoes and chicago bulls.
All are the same size, have light imprints, and are well pressed, but vary in colour from white to pink, green and blue.
They are all pretty similar in effects. Often you will see blue specs which I assume is ketamine. Sometimes there are brown specs which could be XTC coz they sometimes feel quite luvd up (read danCingQueens report on chicago bulls and the debate over this in the comments).
They're not bad if you're into this sort of thing - usually make for a fun chatty dancey night - just don't pay too much for them.