Underfloor heating flow meter brown, why?

Hello everyone,
Today we noticed that in all HKVs in the building, all flow meters are brown. It is hardly possible to see what flow rate is set.
The building is a new construction, from 2019.
Why is this the case? And is this something to complain about to the developer? I have seen that these meters can be cleaned or have cleaning done. But I would rather be interested in whether this is acceptable after 3 years, or if something might actually be wrong here and I should file a defect notice within the scope of the warranty.
Thank you

Was the heating water demineralized in your system? Was a separator installed? Do you have a towel dryer or other radiator connected to the underfloor heating?

In my case, the water was demineralized and a magnetic separator was installed - the flow meters still turned brown. Not as brown as in your case and only where the flow is particularly high or the circuit was always open.