Why Does Minas Morgul Glow Green?

There's no explanation given for the eerie green glow we see in Minas Morgul. It's only described in the book as having an evil, sickly glow and generally being creepy as heck.

What is the light in Minas Morgul?

Barad-dûr had a red flash, Minas Morgul a blue. Tolkien says that Sauron used Magia (visible physical effects) for evil purposes.

Does Minas Ithil become Minas Morgul?

Minas Ithil was occupied by fell creatures, and it changed into a foul, evil place. As a result, it came to be called Minas Morgul, which in the tongue of Gondor means "Tower of Dark Sorcery".