Concise answer
A tan is a pigment response after ultraviolet exposure, while pigmentation includes many patterns such as melasma, post-inflammatory marks and sunspots. Appearance and timing help, but photographs alone may not settle the difference.
A recent even darkening after sun exposure may gradually fade, whereas sharply localised or recurrent patches can have another explanation.
Harsh attempts to remove colour can trigger irritation and make both conditions look worse.
When evaluation may help
Assessment is appropriate for persistent, uneven or changing colour, especially when it itches, scales or follows a procedure or strong product.