Cloud free land depicts in magenta colours in the Night Microphysics RGB images.
Magenta is a combination of the red and the blue colour beam.
For the contribution of the red colour beam, the brightness temperature difference (BTD) of the MSG channels (IR12.0 - IR10.8) is around zero when Cirrus clouds are absent. As the used colour scale reaches from -4 (black) to +2 degrees (intensive red), the resulting red colour is of medium intensity.
When there are no low clouds, the green colour beam gives little contribution. The measured BTD (IR10.8 - IR3.9) are small and mainly due to CO2 absorption at 3.9 micrometer.
The blue colour beam is contributing with an average value. The warmer the surface is, the more the radiation from IR10.8 is contributing to the blue colour intensity.
Cloud free sea has a more blueish colour than cloud free land when the sea surface is warm (see image below).
Night Microphysics RGB from 23 August 2015, 00:00 UTC
Explanation of the magenta colour of cloud free land in the Night Microphysics RGB (see the recipe):
For cloud free land, only the red and the blue colour beams contribute (red + blue = magenta)