interaction design - When is red the right color for a button?

After reading this question about Big Bad Red buttons, where it was generally agreed upon that red buttons draw the user to click, I was left thinking: then when are they good affordance and interaction design?

• 
  

  Red buttons usually mean something negative, so I suppose users will be expecting that of a red button.

  
  


• 
  

  Red buttons draw users to click instead of avert from it.

  
  
  

The two statements above seem to work against each other. So is the red color "out" for buttons? When is it a good idea to use it, if at all?

Answer

Red buttons are appropriate when a primary action is negative in nature.

That is, the user is very likely to want to hit the button anyways, and the action triggered is canceling, ending, deleting, or some other "negative" action. Here are a few examples from iOS:

Note that in the image on the right, the user has already indicated they would like to leave the current page, so they are likely to want to hit that "Delete Draft" button. If that action had been of secondary importance in this context, I would not go with red (and that location and size on the screen).

This pattern reinforces the nature of the action while also calling attention to the place you are likely to want to click/touch given the context.