With Martian Manhunter, what I have is by and large in the character sheet. But a few things I'll lay out here.

-Mars is inhabited by the Green Martians. However, they're ruled by the Martian World Government, a Soviet-style authoritarian government (helped by the fact that they can literally thought police people) though they're able to mask themselves as Utopian to the outside world and the upper echelon citizens. So casual, short-time visitors may not even notice anything wrong right away, but people that plan to have characters living there or stay there long-term should be aware of the true nature of the place.

-White Martians exist, but let's say the Martian World Government hasn't been the kindest to them while they've been in power.

-While I have Martian Manhunter mainly set up in the New York City police force, I'm open to having him borrowed by other police forces for cases and so he's open for collab stories, especially if people want a police detective within their story. Same holds true if you're in New York itself.