As @robertklep already mentioned, only Athom can fix it because this is a user forum only.
Just an additional info: the color black is also used when the actually temperature is the same like the target temperature or the mode of the thermostat is off.
Please have a look at this post (I guess you understand German).
Edit
I have just checked the behavior again myself, Athom does indeed seem to have changed something. But probably not to the Homey firmware, but to the Homey smartphone app.
This may be due to the fact that the climate tile (Smartphone App → Devices) was added with the smartphone app version v8.0.0, which was released in the last few hours.
My thermostats now only display red (= heating up) and black in the WebApp resp. grey in the smartphone app (target temperature is equal to or higher than actual temperature). This also makes sense with my thermostats, as they cannot actively cool.
