Fog over the North Sea and in the eastern valleys of Scotland and England are yellowish/ochre in the Cloud RGB images, because their reflectivity values are high and close to each other in both visible channels, the 0.63 and 0.87 micron channels, while their cloud top temperature is relatively high (close to the temperature of the surface). As the 10.8 micron channel is inverted in this RGB, warm temperature means dark shades in the blue colour beam. Nevertheless the blue colour beam is weaker than the red and green beam. This leads to a yellowish/ochre colour of low clouds.
MetOp AVHRR Cloud RGB image, 23 July 2014 10:22 UTC