[kaukcz (OP)]

Why are nipples always darker than the surrounding skin, and why are the palms and soles of the foot lighter in black individuals?  Is it evolutionary?

[rosalind dna]

Could the reason be that some parts of our bodies happen to see
more of the daylight than other parts.

Like our hands, faces are seen but in some religious cases I know that can be otherwise (not mine). Or as you said our nipples.

Or the different parts of the world that we all come from even on this forum, like I come from the UK.

[kaukcz (OP)]

My best guess in the case of people with darker skin tones and lighter colored palms and soles is that these tend to be higher friction parts of the body and skin cells are shed more rapidly.

[rosalind dna]

Perhaps or it might be to do with the diet and types of food that they eat. Or as I've said where we all live in the world which
can be influenced by the different kinds of temperatures.
E.G. Cold, damp and wet here in the UK now.

[BenV]

The hands and feet have a layer of keratinised (toughened) skin on them, so that would account for the differences there.