No red cheat

No red is a Multiplayer cheat which causes the red-screen effect caused when a player is damaged to be removed or replaced with an alternative effect which affects playability in a less extreme way (such as a slight lightening of the screen). This gives an unfair advantage as the cheating player is still able to see properly when injured.

The cheat usually takes the form of a PWAD loaded with the game which replaces the COLORMAP lump.

Detection of this cheat involves visual observation of the player's screen as the game is played. Because of this, it may be difficult to prove that a player is cheating.

Source

This writeup includes text from Doom2 cheat modification, used with permission.