O2 Test ClayRender

The whole time I was making this I thought of what cities might look like in 160 years

Please. if anybody knows how to add texture to this kind of model.
Help. Tutorials on google have been horrid.

I'm wanting it to look like a dark metallic rusted deep blue or black steal like building
if you follow that with lights and little windows.

I just dont know how to paint it, if anybody would want to work with me on this that knows what they are doing
or knows of a good tutorial please let me know asap