As far as I know there is any jewellery connected to Germany. For the first meeting something neutral, pearls and some flower brooch or garland style (a Bow brooch or Richmond brooch?). Maybe something from QMary? It is reception with military honors.
In the evening state reception: if red stones then Burma Ruby Tiara, although this might be to small for Queen Camilla. So, perhaps red dress and white jewels, Dehli Durbar and Greville Necklace.

It seems that Queen Camilla will not accompany the King on Thursday (speech in the Bundestag, meeting with refugees of Ukraine, meeting german-britisch Pioneer Battalion and a village with organic farming). That is a lot of program. Instead she will went to the Komische Oper with Elke Büdenbender in the afternoon.
Didn´t the late Queen got a brooch in form of a clef?

I doubt there will be time to dress up for the reception of the British Embassy in the evening after the programm in Hamburg. The location has quite an industrial charm and I guess the King and Queen will leave Hamburg immediately after it. So I don´t expect tiara but pearls, maybe a choker with colored brooch, and one of the very big flower brooches.