What are the differences between redshirting, grayshirting, blueshirting and greenshirting in college football?

There are a lot of 'shirts in college football. Here's what they mean.

Understanding the Different Colored “Shirts” in College Sports — Naples Sports Performance Training

What It's Like to Take a Redshirt in College Football, News, Scores, Highlights, Stats, and Rumors

What are redshirts, grayshirts, blueshirts and greenshirts in college football? - Quora

Understanding Athletic Greenshirts & Blueshirts

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What are redshirts, grayshirts, blueshirts and greenshirts in college football? - Quora

Guiding the College-Bound Student Athlete - ppt download

Redshirts, Grayshirts, Greenshirts, and Blueshirts

In college sports, what does 'red shirt' mean? - Quora