Brazil city is colored green (instead of grey)

The residential area of Anápolis in Brazil is rendered green:

Is this a rendering or a tagging mistake?

Yes, it looks like a tagging mistake:

The entire area is mapped as a park - hence the green rendering.

This is the park relation multipolygon:
And this is the outer way of this multipolygon:

On Dec. 5th user Miduim changed a residential area relation encompassing the whole city into a park.

You might want to contact Miduim via that offending changeset what their intentions were.