Replacing dining room Light switch : 4 wires, what is the pink for?
Sounds like this is a three-way switch, or at least wired for a three-way at some point in time.

You will need a different switch, specifically a three-way switch, and it should be wired in the same way as the existing switch.

You should locate the other switch that controls that fixture to confirm its existence.  If it doesn't exist, then the pink wire was probably used as power to the fan, but in that case I would expect this would be in a double-gang box.