Why Black?

[sm78]

And the intersesting things i know is,
The first Deitys i know who standing on Shiva is Kali.
and after that i know in Buddhist also have some Deitys standing on Shiva,
this is very interseting for me, this is Shiva's Lila.

Kali standing on shiva: The dance of kAla (time) and the lIlA of sristi-sthiti-laya-sthiti occurs on the 'chest' shiva who is just a mute witness...mute as a dead corspe.

Buddhist deities trampling on Shiva or Kali is typically to represent the superiority complex of bauddhas vis-a-vis sanatana dharma and typically doesn't have any esoteric significant other than egoistic nastika demonstration of supremacy.