I can't do it!
You need to be more specific with your questions - how do you change the colour for what?
For characters, it's the 'Talking color:' setting, or the Character.SpeechColor (http://www.adventuregamestudio.co.uk/manual/Character.SpeechColor.htm) command in-game. For the various GUI bits and bobs (labels, buttons, etc), there should be a 'Text color:' option you can change. For the normal system text (e.g. with a Display() command), I think you have to use a Custom Text Window (http://www.adventuregamestudio.co.uk/manual/TextWin.htm) to be able to set the colour.
I'm talking about the charecter talking color. So what do you meant by changing it "in-game"?
EDIT:
forget this post. I've figured it out
For future reference:
If you wanted a character's speech colour to change during the game (to represent emotion, or health, for example), you'd use Character.SpeechColor, e.g.:
cEgo.SpeechColor = 10; // Green
cEgo.Say("I'm not jealous ...");
cEgo.SpeechColor = 12; // 'Default' Red