Don't you know the Doctor doesn't tan (at least in this regeneration -- unless you want to see a beet-red Time Lord). It's probably one of those mildly terrifying sights, like the Doctor without his shirt, perhaps a little blinding and hypnotic. Worried less about shirts and sunburns, the Doctor squints at the Master's back, watching as he works with the wreckage of that horse and quite certain he has to be seeing this wrong. Maybe his eyes are going in his old age. Of course that's highly unlikely, what with his brand new body, but it's one of the few explanations that makes sense -- much more sense, in fact, than the most obvious one.

The Master looks like he's constructing some sort of sled, man-sized.

"You can walk," the Doctor points out. Why yes, he does sometimes moonlight as Captain Obvious. His voice is quiet as he studies the Master's back. "So why are you doing this?" In other words, why is he suddenly trying to save him? It looks suspiciously like the Master means to do just that.