@panoramajuerg
As explained by @Gkikas-LGPZ, there are areas showing rain and the color scale gives the amount of precipitation in these areas. To know the cloud cover in % you must use the weather picker.
But if you want to get the cloud cover scale, see Menu > Settings > Customize layers colors > Select overlay > Clouds

B415A00D-75E5-46CA-997B-911877D3D4D1.jpeg

Note there is a mistake in this scale: you should read % instead of mm.
Personally I have modified this color scale to avoid the dark grey used for 30% cover and with more details.

E418F264-14EE-401A-8449-0C4B15EFAB0E.jpeg

If you want to use this setting, just let me know.