These days, the color red doesn't really stir up any crazy emotions. Well, no political emotions, at least. Not so in 1957, when it was closely associated with Communism. And not-so-coincidentally, the Cat and the Things are pretty red themselves—from their clothing to the FUN-IN-A-BOX to the cat's cleanup mobile.
Symbolically speaking, we might see the Cat and the Things as Communist invaders corrupting our youth—in their very homes—while their parents are away.
Now might be a good time to click over to "Symbolism, Imagery and Allegory." As it turns out, the Cat and the Things aren't all that's associated with the color red: it's also the color of the kids' house, the mother's coat, and little Sally's bow. So if the Cat and the Things are Communists, that means the mom and kids are, too. Well, that sure complicates things.