Red means danger, in traffic lights and brake lights. Animals use red to issue warnings and announce their mating season. Red is used to provoke erotic feelings. Red is the color of your blood. Most of all, red means STOP or ERROR.
One example of bad use of the red color is seen in Replicon Web TimeSheet:
The fist impulse we get when we see this is "there's something wrong with those weeks". As a matter of fact that red only means "this week is already filled in".