Exploring total cloud cover, the colour scheme shown on the map does not match the legend in the bottom right hand corner (the legend is also titled "mm", i.e. is for rain/snowfall). I'm using Chrome 58.0.3029.110 (64-bit) on Windows 7.
The correct legend shows up for all other cloud categories (low, med, high, tops, base, etc.), and for other phenomena e.g. wind, temperature.