Gotta catch em all!

Green cats are grunts. They are to take care of regular defenders and create chaos.
Red cats would then be a second wave, to find hidden guards, focus magic users on themselves?
Then will a third wave of blue cats come, they will do whatever they came here for (free the prisoners? capture other kids?)

The pattern on the red bubble is different as well.

Is that a reference to Red Hat?

This is reaching Monty Python levels of sillyness, and I like it.

green is a secondary color, and red is primary. You should be afraid of the primary cat.

this mean that the blue, green and red cats in the banner aren't arbitrary

Honestly, I think that the red one's red because he's dead and bled into the bubble. The bubbles do look like they're primarily liquid, after all.

Red cats aren't necessarily stronger, as this one only managed to bubble ONE measly guard, putting it on par with the greens. If it were that dangerous, it could probably solo at least a small group of guards without any greens.

If this were Tobi's world, I'd suggest that it means that the Red Cat is the weakness of the group and the one you have to take out to stop the collective.

Since it isn't...

I'll say that same thing with less confidence.

@Darius Drake I was about to say the same thing. I think he is dead. It is like a sort of health bar or status bar to indicate the prisoner health in my opinion.