Mark Richman was twenty years old when he talked his way into the opportunity of a lifetime — wheedling his way past a policeman and into the corps of photographers shooting the Beatles' August 21, 1966, concert at Busch Stadium.The photographs he snapped that day, which he says are the only color photos of the Beatles' final tour, have earned him tons of attention from Beatles collectors. (You can see them online here; the one above is reprinted with his permission.)…