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.

The rise of grayshirting as a trend in football recruiting - ESPN

Redshirts, Grayshirts, Greenshirts, and Blueshirts

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

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

Recruiting's color wheel: Redshirt, greenshirt, grayshirt, blueshirt

What Does Redshirt Mean? And The Reasons Why Players Do It?

What Is Grayshirting and Are You at Risk? - stack

The Recruit Grind

2017 Alabama Football Recruiting News & Update: Dead Period Begins - Roll 'Bama Roll

What Does Redshirt Mean? And The Reasons Why Players Do It?

What Does Redshirt Mean? And The Reasons Why Players Do It?

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

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