A large percentage of users just aren’t very aware of color coding in any useful way. Color is great as an additional cue, but you should never rely on it as the only cue. Moreover, Some people (particularly over the age of 40) simply can’t detect some color distinctions because of color-blindness.
Don’t make me think
Being an expert in online usability Krug gathered his experience in a wonderful book. This summary of key thoughts will help you to remember most important principles and laws.