I have a US SPEC RRS. The back windows are darker...is the glass darker or is it a tint film on there? I want to get my car tinted (darker) so I'm wondering if they have to take off the factory tint first or just tint over it (if it's just glass).

if it was spencer with Privacy Glass when built there is no film. The glass itself is tinted. Range Rover Sport SC - Java / Tasmods / Skirts
Morgan Plus 8

An easy way to tell us to look at TRH rear window. If you can see the heating element from outside, it is film. Range Rover Sport SC - Java / Tasmods / Skirts
Morgan Plus 8

Its part of the glass and not like the after market film ....

At least it is on mine as most if not all RRS in Texas come this way Brit across the Pond
07 TATA (LR) RRS SuperVindaloocharged, Buckingham Blue, Stormers, TPMS, Sunroof, Rear Difflock,PDC front & rear, Privacy, Rear twin DVD, premium ice, mudflaps, premium Ivory leather, lined oak, PTI, Sat Radio.

Yep, the factory installs tinted glass not a film - you can easily tell the aftermarket tints as Paulo points out