Metal takes on oxidation colors when it gets up around 500 to 600F. If your rotor is blue and yellow'y straw colored then that implies that you used your rear brake more aggresively than your front.

Depending on the pads you may have overheated the pads and produced a glazed hard surface on the pads which, depending on the materials they are made from, could transfer to the rotor's surface. But a transfer of that sort is fairly unlikely.

Because disc brakes work by friction if you deglaze the pads with some sanding as mentioned and re-install the friction of the pads will soon wear away the colouring on the rotors. So you won't gain much by sanding them as well.

And it would appear that on the long descent that you're favouring the rear brake. Try to even it up. On clean dry surfaces you're in far more danger of using enough front to actually lift and flip the front end on a descent than of locking up the front, skidding and going down. So don't be afraid of the front. And if it's really long and steep get off the seat and get your weight back more.

If road and traffic conditions allows you to do it safely letting the speed come up so the air friction holds you back more will take some of the speed limiting duties off your brakes. Sit upright as possible for this to act as much like a parachute as you can. It means you'll have to use the brakes harder at the bottom of the hill but you'll do so for a shorter time so you actually dissapate less energy in the brakes than if you use them all the way down the hill to hold your speed to a low value. You'll still use your brakes during the descent but in more of an occasional and short way then allow the speed to come up between applications. But if your route has lots of side streets and driveways this likely isn't a great idea. Too often drivers stuff the nose of their cars and trucks out into the curb lane area before even stopping to look at what traffic is doing.