Warum wird Eisennagel rosa?

Because copper hits the nail.

The less noble metal is oxidized and dissolves as an ion, while the more noble metal is reduced and precipitates in pure form. For example, copper is more noble than iron. If an iron nail is introduced into a copper salt solution, the copper is deposited in pure form on the iron nail and the iron is dissolved as an ion. Conversely, a copper nail does not react in an iron salt solution.

The less noble metal is oxidized, emits electrons and the more noble is reduced, absorbs electrons. The whole is called a redox reaction, i.e. an electron transfer. How noble or unpredictable is a metal, you can see on the basis of the voltage series (positively noble, negative unpredictable).

http://www.kathrin-hoy.net/Photos/Spannungsreihe.pdf