Problem with Herpa 2 piece sport wheels!

[h0aircooled]

I stored the 2 piece Herpa wheels for several months and the metal rings on some of them got darker. It was stored in a clean plastic case, with the models ready for assembly... hidden from the light, in a clean room.

It was a shiny new wheels. I just disassembled blue and grey wheels and made some wash with Tamiya acrylic thinned with Tamiya acrylic thinner. After it dried for an hour or so I assembled them and stored.
I took this photo before putting them to the box

On this picture you can see some of the rings are not as shiny as before...


Does anybody know which material / finish is this and if I can somehow bring the shining back? Any suggestion how to avoid such things in a future?
Is it possible that Tamiya acrylic reacted somehow? Or any other possible reason?

Thank you!

[h0aircooled]

1. I wrote to Herpa and they suggested me to try car polish. (Very helpful and fast support btw!)

2. I was thinking what happen and got the answer! Before disassembly and applying the Tamiya wash I washed the whole wheels with dishwashing liquid. Some micro parts of the liquid are probably stayed on the surface after cleaning and the aluminium is being attacked by the dishwasher...