The pink label simply asks the postal worker to ensure a clean cancel because it's a stamp collector's letter.
The second is more complicated, and I'm not sure if the following is 100% correct. From what I've been able to ascertain, however...
For many years, East Germany had an organization called the Kulturbund
, or German Cultural Association, which was supposed to support German cultural pursuits and projects. One of its sub-entities was the Philatelistenverband, or Philatelist's Association. One of its principal goals was to facilitate international stamp exchanges.
At the time, there were limits in East Germany on the value of unused postage that East Germans could send abroad without receiving equal values back in return. These green and white stamps were part of a bureaucratic process designed to monitor the exchanges for compliance with those laws. International stamp exchanges by East German collectors were only possible through the Philatelistenverband/Kulturbund. The outbound letter would be delivered unsealed to an official government office (Tauschkontrollstelle, or Exchange Control Office), with a list of the stamps enclosed for exchange. The value of the stamps would be determined by the Lipsia catalog, which was the standard philatelic catalog in East Germany.
The green and white stamps were used in pairs. One was used on the outbound letter. The other was sent to the recipient inside the envelope, who would use it on the return letter. Upon receipt of the return letter, the East German collector would then have to show that they had received it with the return control stamp attached, and the foreign stamps would be inventoried to ensure they were roughly equal in value to the East German stamps sent in the original shipment.
On your stamp, you can see the cancel of the PHILATELISTENVERBAND IM DEUTSCHEN KULTURBUND (Philatelic Association of the German Cultural Association), and that it was processed at Tauschkontrollstelle 15 B.
The text on the stamp translates as follows: Tauschsendung = exchange shipment, ZKPH = Zentraler Kurierdienst Philatelie, or Central Official Philatelic Courier.Here's
a rough translation of a German Wikipedia article on these stamps, and here's
a page showing more.