Oh please, you can't know that...It's completely obvious that the blue background is some kind of Pocket Dimension...At the very least, it's a better description than "they threw it in cause it's cool". I mean, it's so like the gates it's practically stupid to think otherwise...That's right, BELIEVE ME!!! OR DIE!!!
I think it's red because of Lavos. Ayla calls Lavos "Big Fire" (basically) and then there's the chunk of Lavos which is the Frozen Flame. ie, it's safe to assume (and plus we see him at various points), Lavos appears to be at least partially red. Also, with whatever atmosphere Chronoworld may have, it's safe to assume that Lavos would twinkle and shine, much like any other star. He IS moving through space at incredulous speed, right? Perhaps there's something there that I can't really go into because I don't know everything, but I don't see anything in game that points to it ever being a gate.