This is a pretty general feature which wouldn't impact the gameplay. Basically, it's about a possibility to change what the colors displayed on your screen mean, in order to get some quick, in-context information. This could be done without spending any game time, just like switching tactics.

ADOM uses colors to identify subclasses of monsters. This would be the basic mode. Here are a few more:

Alignment mode: lawful monsters will be shown in white, neutral in light gray, and chaotic in dark gray. Monsters whose alignment you don't know will be displayed in red.

Damage mode: color of monsters (and also PC) will show how much it's injured, say, blue for uninjured, light green for slightly injured, dark green for injured, light red for severly injured and dark red for critically injured. White for monsters whose injuries you can't see.

River mode (from my Rivers thread): showing the depth of water on every square.

Relation mode: showing the relation of monsters towards you (White - unaware of you, Red - aware and hostile, Green - aware and neutral, Blue - pet/companion/slave). Got this idea from the Stealth thread.