You can also find a good description of layers here:
Regarding the "Clouds" layer. Like Idefix said, you have to consider 2 info in the same layer :
Clouds coverage in percent from 0% (sand/gold) to 100% (white) of the sky covered by clouds.
Rain or snow (depending on temperature) accumulation in millimeters for the next 3 hours from the selected time. This varies from 0 mm (no dots) to more than 50 mm (purple/light pink dots).