Sandy desert appears cyan in the Day Microphysics RGB because of the following reasons. The reflectivity of sand in the 0.87 micron channel is medium strong, resulting in a moderate signal in the red colour beam. The reflectivity of sand in the 1.6 micron channel is much higher, resulting in a very high (often saturated) signal of the green colour beam. The temperature of the desert surface during daytime is very hot, causing a strong signal in blue colour beam as well. So the mixed colour for the daytime sandy desert will be cyan (medium red, intense green and blue).
The images below show the Sahara on 04 September 2014. Note, that the bluish patches within the Sahara desert are the areas not covered with deep sand, but rocky fields, rocky vegetationless mountains, or vegetation along the Nile.
MetOp AVHRR Day Microphysics RGB image, 04 September 2014 07:58 UTC
MetOp AVHRR Day Microphysics RGB image, 04 September 2014 08:52 UTC