You notice one wheel's rotor is far rustier than the others - heavily orange while the rest look clean. Since rust gets scrubbed off by braking, one rotor staying rusty means that brake is not working as hard as it should. Here is what it points to.
Rust is scrubbed off by use
All bare-iron rotors flash-rust when wet - see why rotors rust overnight - but the pads scrub the swept area clean on every stop. So a rotor that STAYS rusty across its friction surface is telling you its pads are barely touching it. The odd rotor out is the clue.
A sticking or lazy caliper
The most common cause is a caliper on that corner that is not applying properly - a seized slide pin or a partly-stuck piston so the pad drags weakly or not at all; see sticking caliper symptoms. That wheel does little braking, so its rotor never gets scrubbed and stays rusty, while its pads barely wear - the mirror image of a fast-wearing brake.
A collapsed hose or rear-bias quirk
A restricted brake hose can starve one caliper of pressure so it under-applies - see brake hose replacement. Rear rotors also naturally rust more than fronts because the rear brakes do less work - see why rear pads last longer - so a rusty rear versus clean front can be normal; a rusty rotor versus its matching pair is not.
What to do
If one rotor of a pair stays much rustier, inspect that caliper and hose - free the slides, check the piston, and replace a bad hose. Fixing the under-applying brake restores even use. Then fit fresh pads and hardware as needed.