I have not a clue as to why that might be going on other than something has been reduced and you can no longer use the spell . . . OR in the last patch, adjustments were made to that particular skill or spell and you can no longer use it.
Check with the log-in notes, go back a few patch notes and check carefully.
As to skills going green, I never open mine in or at end of combat to see if skills have changed. The only time I really open them is to see if I can scribe a craft-book or harvest something. I tend to just enjoy the game in RP and not worry about analysis of what is going on . . . unless of course something like what your describing happens.
The stats being Green or Red or White is a thing carried over from EQI but not sure if it applies to the skills, only to stats . . . but I could be wrong.
Red - your stat is no longer where it used to be - either above your base or below it. This applies if you have items buffing your stats: Your BASE is 35, you have BUFFED armor bringing it to 40. You get nailed by a spell and it goes to 38. The stat will still show Red . . . because it is below buffed even though still higher than BASE.
White - stat is at your BASE LEVEL only . . . That shows that you have removed any buffs from yourself, armor weapons etc and your stat is at 0 statistically.
Green - obviously - your stat is above BASE and is being buffed with equipment or spells.
I don't think I am incorrect on any of the stat-warning information.
Good luck
Mooska, Barbarian Crusader
Antonia Bayle