If you have disc brakes all round then what you see is the disc, the hubs are hidden behind them.
You will always see a small film of corrosion from the unprotected metal disc surface that is gripped by the pads. This can become apparent in only a few hours in damp conditions but will be wiped off very quickly by use of the brakes. There is nothing you can do to prevent this.
Good quality discs will generally have some sort of reasonable rust prevention or paint finish applied that should serve without rusting until the discs are worn and replaced.
Cheaper discs often skimp on the corrosion treatment of the centre and can become rusty quite quickly. you can paint the centres (best done from new) if they are already rusty you need to remove the rust as much as possible with a wire brush, wire brush in a drill/angle grinder, coarse wire wool, abrasive paper etc, wipe down with solvent cleaner and then spray with a zinc primer aerosol and then the colour of your choice.
You can also use a metal anti corrosion paint such as hammerite/smoothrite.
If you paint by brush especially you should be very careful to apply thin even coats and avoid runs. The disc faces mate with those on the hub and wheel and thick runs. blobs debris etc may prevent them mating properly and in extreme cases cause run-out and wobble/balance issues