If you touch a pink reader and there is a fare defined between your origin and destination using that pink reader then you will be charged the defined fare. If there is no fare defined then you will be charged the appropriate fare for that journey taking into consideration any other interchanges you have used.
The Oyster system can’t tell what you are going to do in advance so a red light if you end in zone 1 could never happen. Besides, there are a few instances where a pink reader can make a small difference to a journey with one end in zone 1 (eg Brockley to Farringdon).
As alluded to by Colin and Simon, there are a few cases where it’s possible to use a pink reader and still travel via zone 1 without paying extra. This is not a topic that I want to discuss on here because it’s not within the spirit of the rules for using the Oyster system. In many cases the walking required to touch a pink reader if trying to get back on the same train makes the effort too much for the gain.