Is there anywhere on the site where you can view all the pet colors any given species is available in? Something akin to the Rainbow Pool (I'm not sure if we can mention offsite places on here...)?

There's nowhere onsite but Subetalodge has a guide

Thank you!

you can mention that inferior virtual petsite here if you like, you just can't promote it. Subeta favors freedom of speech as long as you are nice, and you realize the site is PG-13. Also unlike that place the Rules here are available for you to look at and staff will respond reasonably quickly, politely and accurately if you file a Ticket in Help. There's a set of links at the bottom of each and every page (they're itsy bitsy, but they're there).

Neocolours sometimes does Subeta colors too, and tends to have larger images last time I looked years ago.