i've seen a couple of mods change colors for spells', active effects' etc menue displays -

anybody got any idea how to do this?

(tried text tagging like for books in descriptions, but the tags themselves just displayed as plain text)

Am not familiar with the Creation Kit but there might be color controls where you can change such things or perhaps use something like photoshop to change the spell colors then import them into the game world with the creation kit. I have seen and used a few mods for OBLIVION that did this like the ones done by a Modder called Pepsi. Maybe you could download a similar mod and take it apart to see how others have done this.

Best wishes