Color of ants in A Bug's Life (1998)

Most likely because realistic ants didn't look kid-friendly. This tor.com article discusses this somewhat:

Cute and friendly bugs proved difficult for two different reasons. For one, these were, after all, bugs, and for two, the Pixar rendering systems were still not capable of creating “soft” looking, variant textures, but only hard, shiny, plastic like surfaces. In many ways, that plasticity was ideal for portraying the often smooth exoskeleton of ants — but unfortunately, the more the ants looked like ants, the less friendly (to humans and small children) they seemed. The usual cartoon touch of just creating bigger eyes, often associated with “cuteness” (what Disney had done, well back in the day, to make Mickey Mouse look “cuter” and more approachable) also tended to backfire, since large eyed ants on a huge movie screen looked, well, like rather terrifying giant ants with unusually large and scary eyes.

Stuck on both the eye and surface rendering problem, the animators tried a different approach: making the ants more human looking. Instead of the standard six ant legs, for instance, the ants in A Bug’s Life have two legs and two arms—and fingers and toes. Even more importantly, they walk—or scurry—upright—making them look a little more like humans dressed in bug costumes than actual bugs.