From ZDoom Wiki
void A_Log (string whattoprint [, bool local])
Logs a string to the console and displays it on the screen.
- whattoprint: The string to log.
- local: If true, the string is logged if the player is either looking out the calling actor's eyes, or the calling actor is the player and the player is looking out the eyes of a non-monster actor. Default is false.
TNT1 A 0 A_Log("Hello World")
This will log the above string, with Hello colored in red and World in blue.
TNT1 A 0 A_Log("\cGHello \cHWorld")