[Lazy_Perfectionist]

Having only cursory knowledge of Warhammer, I offer the following descriptions. I'm certain they could be better, but I'm trying to sleep before 2:00 AM. Maybe they'll provide some inspiration. As well... I have the feeling that they aren't quite the flavor for the Forces of Chaos, from what I do know... But if I was hijacking those spells to add to all nations, here's how I'd add em. I'd let someone who owns fluff material come up with nation-specific descriptions.

Red Fire - A spiritual flame, it burns unbelievers.
Red Fire - A spiritual flame, it burns those weak of mind and faith.

Yellow Fire - This flame burns away the doubts of the believers and makes them strong in their faith.

Green Fire - A sickly pale green fire that devours the sanity of your enemies.
Green Fire - A sickly pale green fire that consumes trust, causing friends to turn upon one another.

I hope this was of use, but if it missed the mark.. no big loss of my time. Best of luck with the mod.