http://en.allexperts.com/q/College-Footb...-Shirt.htm
Grayshirt
is another new term and is applied to the prospect that signs a letter of intent in February, but doesn't report in the fall with his teammates. He delays entry to college until midyear, i.e. January. That NCAA five-year clock doesn't start ticking until the player enrolls as a full time student, so gray-shirting is really a delayed version of red-shirting. For Example, Texas Tech, signed 34 players in February 2006, but NCAA rules prevent them from enrolling more than 25 to start the fall. Some of those nine other players gray-shirted during the 2006 fall season. They cannot enroll in college as full time students, can’t receive their scholarship, nor practice. It is like getting an extra year of practice, because most of these players don’t see the game field until two years later and they have the advantage of going through an extra spring practice.
Red shirt,
the most well known "shirt" color has been around for a long time. Red means “stop” as in “stop from playing”. The NCAA allows a player five years to complete four seasons of eligibility. That fifth year, usually the freshman year, is when the player practices but doesn't play in any games. That is called the Red shirt year. Players still receive their scholarship, still practice, still do everything the other players do—they just don’t play in the games.
Green shirt
is a relatively new term and is applied to that high school senior that forgoes his spring semester in high school to enroll in college in January. Green means “go” as in “go early”. More and more you see high school seniors pass up their final semester to get to campus early. If a player (or the college staff), thinks he may be able to play right away as a freshman, it provides him with the opportunity to go through spring practice, learn the system, and get acclimated to being on campus. Obviously that takes some prior planning on the part of the player and his parents to insure he can graduate from high school early.
http://www.al.com/sports/index.ssf/2013/...luesh.html
http://www.knoxnews.com/sports/vols/foot...97791.html
Blue shirt
With the new rule interpretation allowing early enrollees to sign as early as August, it means those athletes won't be counted in the current year's class as long as they fit within the NCAA limit of 85 scholarship players on the full roster.
Blue-shirting is an approach pioneered by New Mexico State, which used it recruiting junior college players. The NCAA allows athletes who were "not recruited" to arrive on campus as walk-ons, accept scholarships at the beginning of preseason practice and be counted against the next recruiting class instead of the incoming one. An athlete is only considered to have been recruited if he takes an official visit to campus, accepts an in-home visit from the coaching staff or signs a National Letter of Intent or other athletic-based financial aid agreement before arriving on campus.
By blue-shirting an athlete, teams can take in more than the 25 scholarships allotted each season for "initial counters." Programs can have 85 players on scholarship at any point in time, but are only allowed to have 25 players as part of any individual signing class. Blue-shirting allows some relief from that. The Vols took advantage of the loophole last year and are expected to do it more this year. Vincent Perry, an all-purpose back from Hillsboro High School in Nashville is also expected to blueshirt.
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Catch up on all the latest news on UAB football recruiting
Catch up on all the latest news on UAB football recruiting 
