How do color swings/color differences work? I started my first project, and I noticed that the AC1 and AC2 colors are very different in tagged pets of the same theme (e.g. this one, this one and this one). I don't think the colors were changed, otherwise they would have an icon(?)

Ooh, that makes sense. I didn't know accent colors were added recently. That might be it since they came before they were added. Thanks!

To add to the before and after revamped pets, rng (random number generator) can do some pretty wild color swings as well. I've seen themes with three different colors in the same color slot (I believe it was supposed to be white, but there were some with soft greens and soft yellow hues.) You will find that it is usually the pastels that do this opposed to the brighter colors, but there will always be slight variations thanks to rng. Below you can see where there is a slight color swing between the two (the male is slightly more yellow than the female's white.

I find color swing hilarious sometimes. Yeah, it can be annoying, but it usually averages itself out if you're doing a project. Like Shadow mentioned, it's most noticeable in white/pastel colors because it doesn't take much to completely change the hue, a perfect example being the SB Rikkis I've been seeing. The left is the display colors from the themed pet list, and the right is an unfortunate victim of color swing. The pink turned a white-ish purple, and the white turned green.