What can cause a dark skin pigmentation on my neck?

Many things. Acanthosis nigricans causes dark, velvety patches of skin on the neck & skin folds and since it can be associated with diabetes or other health issues, it is important to have it looked at. Many other causes exist: sun-exposure, post-inflammatory hyperpigmentation, tinea versicolor, other rashes & even retained dirt (terra firma-forme) are just a few! See a dermatologist.