Cloud A - Stratus cloud
Grey on the IR photograph means this is a warm low altitude cloud and a strong emitter of IR radiation
White on the VIS photograph means this is cloud is thick and a good reflector of visible light.

Cloud B - Cumulonimbus (thunderstorm) clouds
White on the IR photograph means weak emission of IR radiation coming from a cold high altitude cloud (the top of the Cb cloud)
White on the VIS photograph means the cloud is thick and a good reflector of visible light

Cloud C - Cirrostratus cloud
White on the IR photograph means this is a cold high altitude cloud that is not emitting much IR radiation
Grey on the VIS photograph means a thin cloud that is a poor reflector of visible light (must of the light shines through the cloud)