Octopuses change colors, so most of them could be potentially very colorful. I had an Octopus cyanea that would be bright red with white dots when he was excited or angry, brown and white mottled when he was sneaking and hiding, and bluish green when he was scared. Almost all, or maybe all, octopuses are somewhat venomous, even if they aren't dangerous like the Blue Rings. They can all bite, although they don't bite easily. And, they are all escape artists, which is why I mentioned having your tank set up for one before you get it. As Mike says, they are astoundingly strong for their size, and can pry off tank covers and lift weighted lids to climb out onto the floor. The best method I ever found for keeping them in a tank was to put a strip of astroturf around the top, since they don't like to climb across it or touch it. However, when I dumped a second one into the tank with the first, he immediately climbed out over the astroturf, so it's not like they CAN'T climb over it, they just don't like to. Keep reading and researching, they're neat pets if you know what you're getting into.
Cheers,
Don