Which spice gives brown colour to Indian curry?
I've seen a lot of different curry recipes with varying levels of authenticity, but the most common ingredients I see in curries that might impart that colour are:


Garam masala (brown)
Chili powder (red)
Cumin (brown)
Paprika (red)
Tandoori powder (usually a mix of masala, cumin, ground red pepper, fenugreek, and others - very red)
Saffron (red)


Still, it's all kind of a moot point, because, in Indian restaurants the most common source of red is actually red food colouring.  Don't kid yourself; Indian restaurants use plenty of "artificial" ingredients - they also usually use food colouring in the saffron rice, which is how they get those few vibrant red grains to mix in with the yellowish rice.