Am I right in thinking that a) discolouration is exacerbated by incorrect fuel/air mixture and that b) some bikes have double skinned pipes that don't discolour?
Yes. An over lean mixture can cause overheating with high exhaust temps which in turn are likely to turn exhausts blue. An over retarded ignition timing can do similar things.
In reality all pipes go blueish with normal operating temperatures especially close to the head where they are highest. Some bikes have double skinned so the inner skin discolours but the outer stays ok, Harley has covers that do the same.
