There are currently four possible colors for snails: green with red antennae, grey-purple with orange antennae, orange with yellow antennae, and finally brown with blue antennae.
Since the Rusty Snail only comes in orange and the Silkbead Snail only comes in grey-purple, those two colors were already kind of decided for me. That left the green and brown version. Luckily, Scooter the Snail is green so only one color was left to find.
The only snail that has a possible brown skin with blue antennae is the Rapana Whelk in Dread Wastes. It's not the only color it can come in, though, so I had to fight my way through many non-brown, non-rare whelks.
I eventually found one as a secondary, and it turned out to be a color-changer. I'm not complaining, though. :)
It will take a few summons to actually get the brown color that I want, but I'm ok with that since I'd rather keep color-changers and just spend a few extra moments summoning and dismissing it, than getting rid of color-changers altogether.
Besides, I can always head back and try to find a rare non-color changer if I ever get bored or feel absolutely compelled to do so. :P
If Blizzard actually fixes color-changers so they don't switch skins upon summon in the next patch, I might have no choice but to head back and pick up a new Rapana Whelk. Unless of course, if my current pet stays the correct brown color even with the fix.
Anyway, now the color challenge for snails is complete! Next up... scorpions..? XD
Sidenote: For anyone that would also like to try collecting every color of a certain model, all in rare quality, Warcraftpets.com is a HUGE help. It shows similar pets with the same model as well as all the possible colors for each pet. :)