Take a look at GUISkin

You can set the look of your labels using the current GUI skin if no style is set on your label.

If you style the label with a copy of GUI.skin.label you can just change the textColor of the particular style mode(s) you want

GUIStyle localStyle = new GUIStyle(GUI.skin.label);
if (somethingBad)
{
   localStyle.normal.textColor = Color.red; 
}else{
   localStyle.normal.textColor = Color.green;
}

that's sort of the quick and dirty way but it works very well most of the time. The 'correct' way is to use custom styles and skins. Many times our buttons don't have text per se so we have to define a separate style with the altered color text.

also GUI.Color, GUI.BackgroundColor, and GUI.TintColor.