Discs can go light rusty brown overnight in damp weather. Go for a short run using the brakes and see if that clears it up. Get a small mirror and check the inside of the discs.
It is unusual but possible to have 4 seized calipers only braking on the inner surfaces. If the discs are still rusty after the run then you have a problem, either calipers or no pads.
Are the discs smooth or ridged?