Hello. You forgot to mention which part of your neck. Front or back? Pigmentation back of the neck is called acanthosis nigricans. Melasma is common problem. Cause for that is mainly hormonal and sun exposure.
Depending on the extent there are several modalities of treatment of melasma
Next Steps
Take a picture and send your doctor for online consultation
Health Tips
Using a broad spectrum sunscreen ( photostable) will help to stop worsening of melasma