Because in Hitler's B&W world, if fungus had a color, it would be that gray.
Fungus (or rather fungus like behavior), for Hitler, was the ultimate evil.
Because the fungus dissolved and joined things that should be always kept conceptually and physically separate.
As when they create a jelly like softness in a damp corner of a house , something that is neither dry firm wood or wet water but rather a mixture of both, rather like a quaking swamp created from dry firm soil and wet yielding water.
Human fungus, said Hitler, he meant mostly the Jews, did the same to the hitherto separate human races and thus had to be all destroyed ,right down to their spore like babies, if the human ecosystem was to be saved.
Hitler didn't like the negroid blues, but he too, felt he was on a mission from God......