Gloves are tricky, not sure about the NFL but in college there are rules pertaining to the colors allowed for gloves so referees are better able to see hand placement, holding, etc. If a team is wearing a blue glove and playing a team with blue jerseys I could see it being an issue.
another story on it:
http://sports.yahoo.com/nfl/blog/shutdown_corner/post/NFL-players-may-be-fined-for-wearing-unauthorize?urn=nfl-wp6799