Thick ice clouds are whitish in the Cloud RGB images, because their reflectivity values are high and close to each other in the 0.63 and 0.87 micron channels, while their cloud top temperature is low. As the 10.8 micron channel is inverted in this RGB, cold temperature means intense signal in the blue colour beam.
The images below show deep convective clouds over the coast of Italy and Spain and over Bulgaria. One can see the cloud top features due to the shadows: e,g, overshooting tops, gravity waves. In the shadows we see darker (grayish) shades.
MetOp AVHRR Clouds RGB image, 26 June 2014, 08:46 UTC
MetOp AVHRR Clouds RGB image, 24 June 2014, 09:28 UTC
MetOp AVHRR Clouds RGB image, 03 September 2014, 08:19 UTC