Thick opaque clouds appear white in the VIIRS True Colour RGB images.

RGB Color Guide
This guide will come in handy for matching colours in RGB images with cloud type or surface feature. It also works the other way round: you can select a meteorological phenomenon and ask for the colour that shows the phenomenon in a given RGB combination.
The collection contains the standard RGBs created from the MSG/SEVIRI imager and also some RGBs from the VIIRS and AVHRR imagers.
To analyse the colours of an RGB image, an interactive tool can be downloaded from the "All Resources" pull-down menu (RGB Colour Tool).
The descriptions of the recipes of used RGBs can be found under the "User Manual" menu (RGB Recipes).
RGB Color Guide
Thick fog or low clouds consisting of large droplets appear pinkish grey in the Night Microphysics RGB.
Thick fog or low clouds consisting of small droplets appear light green in the Night Microphysics RGB.
Thin fog or low clouds over land appear pinkish grey in the Night Microphysics RGB.
Thin fog or low clouds over sea appear greyish blue or medium blue in the Night Microphysics RGB.
In the Night Microphysics RGB, warm thin dust clouds over a cold surface appear cyan.
Fresh, cold, continuous snow appears yellow in the AVHRR Cloud RGB images.
High-level, thick ice clouds with large particles on the cloud top appear red-orange in the MetOp AVHRR Day Microphysics RGB images.
High-level, thick ice clouds with large particles on the cloud top appear red-orange in the MetOp AVHRR Day Microphysics RGB images.