How to get rid out of the dirt on the neck. it looks very bad when there is a difference between colour of my skin and colour of my neck?

Color is not dirt??: You may have a pigment coloration change that maybe related to an endocrine abnormality. Acanthosis nigricans skin disorder characterized by hyperpigmentation and hyperkeratosis, occurring mainly in folds of skin in axilla, groin, and back of neck; occurs in diabetes and poly cystic ovary syndrome.

Depends on cause: Try isopropyl alcohol. One of the skin conditions that cause darkening of the back of the neck will be removed this way. If this does not work, a dermatology consult is in order rather than attempt to bleach, to seek a possible underlying cause.

Ring around collar?: This may be a normal variation in skin pigmentation. You can try a facial scrub on your neck.