CS:GO Color List:
Default: \x01
Dark Red: \x02
Purple: \x03
Green: \x04
Light Green: \x05
Lime Green: \x06
Red: \x07
Grey: \x08
Orange: \x09
Also, it seems csgo has an issue with coloring the very first character so you need to use a vertical tab like so ...
PrintToChat(client, " \x01\x0B\x03This works ...");
PrintToChat(client, " \x03This does not work ...");
Also, by adding the vertical tab it kinda makes the chat look a little weird due to the random space at the beginning, so maybe make this tab an option too?