I am fairly new to photo-based art, and would like to know what or how exactly are skin and hair color charts used for? Please excuse my ignorance!!

There are no silly questions--aside from the unasked ones! :D

Skin and hair charts are useful when you are retouching or restoring an image for a variety of different reasons. If you are colorizing an image, they're a helpful reference, for instance...or if you're trying to adjust colors on an image--and you think your skin looks just about right...you might want to sample it, and compare the number values against a sample in the skin chart. What looks good to your eyes (which are GREAT at compensating for color casts!) might contain a heavy blue or magenta cast...and while the numbers alone might tell you that...comparing it to a sample can help your eyes see how far off you are.

Then there's just painting--pure and simple. Sometimes you might need to paint in someone's skin or hair...and if you can sample from their existing skin or hair--great! But if you can't, for some reason, it's helpful to have a palette of skin tones, or hair tones, to select from. In this image, http://www.pbase.com/briarrose/image/41904718 for example, I painted in most of the hair on the side of her face--our right. To keep the hair looking real, I sampled from her real hair...but if I didn't have ANY data, then I could either search through my archives for another image with similar skin and/or hair--or I could pull out my handy-dandy chart, and sample directly from it.

A PERFECT example of the usefulness of samples is this challenge I did recently: original: http://www.pbase.com/briarrose/image/53969639/large . In the end, there simply wasn't enough data to completely recreate her skin perfectly--so I had to paint it in--using a combination of her skin, and samples from my palette: http://www.pbase.com/briarrose/image/54006771/original Take a loook at her eyebrows. I painted them completely from scratch--without any data about their color. I relied on colors sampled from actual eyebrows--and painted. Likewise, with the bags under her eyes--I simply picked up colors from my palette that were from actual "eye bags".

I'm sure other people use them in other ways, as well--everyone has a different workflow!--but that should give you an idea of what people might be doing with 'em! :D