What tonal value should you use when drawing skin? This is as impossible to answer as the related question from painters, “What color is skin?” This is because the values change so dramatically, according to the light, environment, and the ethnic background of the subject. Don't look for a blanket solution. Just observe your subject, noting all the values you can see. Be sure to observe small areas, from highlight to darkest shadow, and represent these accurately. Don't be afraid to use very light or very dark values on skin, no matter if you are drawing an albino or an Afro-Caribbean. If it looks dark, draw it dark; if it looks light, draw it light!
When doing a value drawing of a baby, use a very smooth paper, as the coarse texture of sketch paper doesn't lend itself well to the fine, even values that are needed for a baby's perfect skin.
When drawing adults, observe how the wrinkles on their faces follow the form of the skull beneath, and be careful to draw them accurately. Be sensitive to the vanity of your sitter—some might prefer a more subtle rendering of laugh lines, however charming you find them as an artist! You can draw freckles, moles, and other marks as well; this depends on whether you are drawing a structural, modeled sketch or a value drawing that focuses on surface detail.
When traveling, make sure you are aware of any customs with regard to image-making and portraiture. Some religions, for instance, forbid or disapprove of portrait images, while some cultures find representations of people who have died to be offensive.