it could be from sun exposure fragrance or any underlying even subtle irritation that causes inflammation of the skin can leave behind discoloration also known as hyperpigmentation. there is also a type of yeast infection called tinea versicolor that can leave your neck back and upper arms looking blotchy. your dermatologist will be able to help clarify and offer you the proper treatment to clear the condition.
i am a fair skin afro-american male and i have hyperpigmentation on my neck and arms. what can be the cause of this?