Thats what happens when you break a engine in,the headers should have came with a warning not to use them on breaking in ,because of discoloration.All the ceramic coated headers say do not use for break in because of the high temps associated with this procedure will discolor the finish,not sure about stainless?
Cherry red glow is good ,white hot is not
I have no answer on the lean condition.........might want to check the plugs for that.