That's the atmosphere for Ultimecia. But this island-isolation is supposed to have a different atmosphere since it's part of Squall. The sky was grey at some point, then it turned green-ish. I don't think green is the right colour to express Squall's conscience. However, if the green colour came from Ultimecia's castle, it would make sense.

Also, look at this:



Same colour.

Is this some sort of power source?