Discoloration is usually from inconsistent heat transfer. The colored spot in your picture is due to heat, not lack of rotation. There is likely no contact between the valve and the seat at that spot and therefore it is getting too hot (valve can't transfer its heat to the seat in that spot). This is from abnormal wear or a bad valve grinding, not from lack of rotation. Sometimes rotation can help to prevent this type of wear, but not always. In your pic, the valve likely has the deformity and therefore rotation keeps the heat in the same spot on the valve. If the seat has the deformity and the valve was rotating, the whole valve would eventually be burned. A picture showing the open gap would tell you if either or both of the surfaces are not making contact. Any non-shinny part of the valve face or seat face indicates poor contact.