Division I CFB Recruiting -- the Shirts (Red, Green, Blue & Grey)

Found this article on the different shirts at SBNation.com: What are the differences between redshirting, grayshirting, blueshirting and greenshirting in college football? - by Kevin Trahan Updated Feb 1, 2017

Here's the chart from the article summarizing the different shirts used for CFB.
NOTE: Only redshirts count against the current year's scholarship totals.


Here's the basics from the article. My minor rephrasing includes some commentary in spots on KU usage, some updates on the redshirt rules, and I've sorted the shirts by what we see most often at KU. (I included this rewrite here in case the article itself is someday taken down).

REDSHIRT
Most common, count against current year scholarship totals. A redshirt player is on scholarship and can practice, but does not play, in exchange for an extra year of eligibility (five years to compete in four seasons). Many college football players redshirt their freshmen years in order to get physically ready. Other players redshirt due to injuries, known as a medical redshirt.

According to the article these are the rules a player must follow in order to obtain a medical redshirt (true in 2017, but today these apply to all college athletes EXCEPT FBS/FCS football players; still presented here to provide the best picture of the state of the rules in general):

• The injury must be incapacitating, a season-ending injury.
• The injury must occur prior to the start of the second half of the season.
• The student-athlete must not have competed in more than 30 percent of the season or three contests, whichever is greater.

Note that since Fall 2018 FBC/FCS players may participate in any four games in a season and still use a redshirt that year. See this Sports Illustrated article from June 2018. The change is documented in NCAA Bylaw 12.8.3.1.6 Exception—Football. [FBS/FCS] In football, a student-athlete representing a Division I institution may compete in up to four contests in a season without using a season of competition. (Adopted: 8/8/18 effective 8/1/18 for competition that occurs on or after 8/1/18, Revised: 6/12/19)

The NCAA can force an academic redshirt on a player in their freshmen year if they don't meet certain academic requirements from high school.


GREENSHIRT
Usually referred to as "early enrollee", they count against next year's scholarship totals. A player who graduates high school in December of their senior year and opts to enroll in college for the Spring term. Athletes who greenshirt still have five years to play four seasons, so they can still redshirt.

There are many benefits to greenshirting. These players can get ahead on their classes. They are on full scholarship in the spring and are allowed to participate in spring practice and work out with the team. Greenshirting has become much more common in recent years, as players are looking to get college-ready quicker.


BLUESHIRT
Blueshirting is a new practice, essentially another loophole to get around the annual D1 CFB limit of 25 signees per recruiting year. The blueshirt rule allows schools to put "unrecruited" athletes on scholarship once they arrive on campus, but count them against the next year's scholarship total as long as they don't play. David Beaty used blueshirts at KU to try to build up the Charlie Weis-depleted roster (after taking over at KU in December 2011, for a variety of reasons Weis kicked 29 players off the team in early 2012, and as of Fall 2019 KU has yet to get back to the allowed maximum 85 scholarship players on the roster each year)

Here's what it means to be "recruited," according to the NCAA:

Was provided an official visit to the campus, or
Had an arranged, in-person, off-campus contact with a coach, or
Was sent an NLI or other written scholarship offer.

Coaches can contact players and have them on campus for unofficial visits — when the recruits pay their ways — without technically recruiting them. Essentially, as long as a recruit doesn't take an official visit or host one of that school's coaches, he wasn't "recruited" by that school.

GREYSHIRT
A greyshirt is when a team offers a player enrollment on scholarship at the start of the second semester, after the upcoming season. The athlete then has five years to play four seasons, with the ability to redshirt at some point. Counts against next year's scholarship totals.

Athletes who greyshirt are allowed to enroll as students. They go to class for the first semester as part-time students, either at the school or at a junior college, without starting their eligibility clocks. Then they enroll as full-time students on scholarship in the Spring term.

Greyshirts have commonly been used by programs that oversign, which means not having enough roster spots for all of their commits. Sometimes, coaches are up front about grayshirting from the beginning. However, other times, they will force grayshirts on athletes at the last minute, leaving them unable to find schools that will allow them to play immediately.

Greyshirts can be revoked and turned into regular offers due to unforeseen roster changes, as long as the annual recruiting limits (25 new starts per year, 85 on the roster at any one time in a year) are met.


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ADDENDUM
With the exception of redshirt these different shirts are basically only talked about for CFB as its regular season is limited to the Fall term, making the other shirts possible.