What is the difference in skin color between the palms of hands and lips;what is the difference in the skin color between the genitals and the soles of the feet;what is the difference in the skin color and thickness in the palms of the hand of a person who does heavy manual labor and one who does not?

I think the palms of the one who do lots of manual labor is thicker... because the skin is adjusting to the things that is happening, it develop thicker skin so that the hands will be protected... unlike hands that do not do much work... they just tend to be thin and soft... just like a person, the one who does a lot of job looks streesed out while the one who does less looks relaxed :)

This applies to the sole of the feet too. If you work a lot I suggest you pamper your skin well... and apply lotion so that it will still be soft and won't thicken a lot :)