Arachspearians are arachnids that, despite their use of words, aren't sentient. For some reason, their language sounds exactly like various Shakespeare works. Maybe it was due to some magic tinkering, maybe its just coincidence, who's to say. However, they are still only about as bright as a goldfish.
The different colours seem to reflect which type of work they tend to recite. And in actuality they can only recite a certain amount of lines at most. So sometimes one ends up with one who quotes a really obscure verse. (For comic reasons, Amber chose particularly well-known verses for the Arachspearians that appeared in the comic.)
The only other unique thing about them is that their bites cause completely random reactions. Sometimes it does nothing, sometimes it causes illness, sometimes it causes one to learn ice-magic or for their nose to turn magenta. Some like to play a game similar to Russian roulette with a pet Arachspearian. Not recommended though.
This article currently is directly based on this forum post by Amber.