"The perceived and actual properties of the thing, primarily those fundamental properties that determine just how the thing could possibly be used. [...] When affordances are taken advantage on, the user knows what to do just by looking."
The design of everyday things - Norman
A few days ago, while returning from a trip to London, I stumbled upon a good example of affordance applied to recycle bins.
Colours just make it obvious that each bin is for different things, but since there's no common color-code to identify each kind of waste, they aren't of much help here. The nice thing is that the designers used openings to help convey the supposed usage.
(I especially love two things: the flat paper opening and the choice of having two round openings of different size for bottle and generic.)