Welcome to HCM.
There is no corelation between darkness around the neck and Copper T. Neck lines occur due to weight gain and increased sweating causing more debris to collect in the folds of neck and also exposure to hot sun. I suggest you keep the area clean and dry as much as possible. If still lines persist, get in touch with a skin doctor for skin lightening cream. Coming to your weight gain, most probably it is due to lack of exercise. As the age progresses, the basal metabolic rate
of the body decreases meaning the body starts burning calories slower than before. So to keep the BMR constant we have to start some form of regular exercise. I suggest you calculate the number of calories you are supposed to eat and start a regular exercise routine. Thanks.