Inside of elbows and back of neck are darker than rest of body.wut can i use to lighten them? preferably over-the-counter

Darker skin: There is no easy way to lighten these areas. You can use sun block generously because with less sun, in time you will produce less melatonin which is the pigment that gives darker color. Topical hydroquinone can also reduce darkness but is expensive and requires long term application.

Dark areas of skin: May be an endocrine problem. Please see a physician and exclude this possibility first. Your physician can refer you to a dermatologist, if appropriate. Acanthosis nigricans on the neck as a dark velvety rash often is associated with metabolic syndrome or glucose intolerance and even diabetes. Be safe and healthy first, then handsome.