Discoloration in the parquet, who has an idea?

Such clearly defined color differences are typical for areas that have been exposed to different UV or dirt effects for a longer period of time (under a protective fleece), for example. It is unlikely that this occurs from one day to the next, but it is quite possible that it becomes noticeable from one day to the next. The underfloor heating does not cause color differences on the wood.

Thank you for the additional information. Clear and unambiguous is the statement that the influence or influences for the straight-line shading over several elements must have taken place on site, in your house. Because the individual elements can hardly be laid from the packaging in such a way that this straight border line can form. Such discolorations, let's rather call them "shadow effects", are either due to sunlight on the parquet wood (which you have already excluded) or due to moisture. Now you will ask: Why moisture? It is to be suspected that a covering of some kind lay on the parquet over a large area, which unintentionally acted as a vapor barrier. Such phenomena, as now present on the parquet surface, however, are usually not permanent and neutralize themselves in everyday life. So just wait for the next 1 to 2 weeks and then report again what or to what extent something has changed. Greetings: KlaRa

Hello KlaRa, I have also suspected the moisture, especially since it occurred after turning off the underfloor heating. We will observe this and then see. We turned the heating on at a low setting for now. Thank you very much for the information. Saludos

Please keep your heating on until you move out!!
Additionally, you ventilate the new building three times a day

Please keep your heating on until you move out!!
Also, you should ventilate the new building three times a day

Hello,
It is not a new building but a renovation/conversion of an old textile factory into residential loft space. The conversion has actually been completed since about October 2019, i.e. the interior work. Is heating and ventilating still necessary then? I think it can only be about the screed, but that should be dry after 5 months, right?

Heating and ventilation are always necessary, even in 8 years.