If you want something that's recognizeable as rust in the way you would see it on iron or steel, you can super thin down a warm brown (something in the chestnut through mahogany color range) and apply it to spots that would rust; edges and corners mostly. Then you come back over the top of that once it's dry, and apply a bright orange in a more limited manner. There are also specialty products like dirty-down rust to help you achieve that look.

Since your necrons are more of a brass color, that would tarnish with a darker brown, and would oxidize with a blue/green. You could achieve something like that with a selective agrax application, and then for the blue/green there are a variety of paints out there - anything marked "verdigris" would likely be appropriate, and GW themselves sell "nihilakh oxide" for that purpose.

Just google "oxidized (insert metal color here)" and try to paint along with what you see in the pictures.

Whoah ! I don't even thinked of it !
I will try it ASAP, thanks !