Scale of clouds
When "clouds" selected a wrong scale is given: mm instead of % with blue (1.5mm) to pink (30mm). It seems to be the "rain, thunder" scale.
All specific cloud levels (high, medium, low) work properly.
When "clouds" selected, you see 2 layers:
total cloud cover (%) in shades of gray + precipitation amount (in mm) in a color scale (blue to magenta).
(without indication of the type of precipitation (rain, snow, thunderstorm etc)
When "rain, thunder" selected, you see the amount (rain in mm or snow in cm) in color + the type of precipitation (with symbols)
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
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.
If you want to use this setting, just let me know.