The black one is the unlimited edition, the colored ones were all limited. The golden one was released when the single reached 1 million sold copies (Hmm, bad memory here, maybe it was to celebrate something else) and was also limited. There's 2 differend gold versions that was released for 2 different reasons.
Bah, why was it such a long time ago I stopped collecting Ayumi items, I knew everything about it if you'd have asked 2 years ago... I even knew how many copies was issued of each
.
I'm sure you can dig all the information up on AHS, if you're lucky with the search engine.