i dont know how it is on pshop these days. i use psp. but it's most likely somewhat similar. every paint type brush has certain qualities. you've got size, opacity and so on. density would be similar in the real world to how many bristles on the brush. if you were using the airbrush tool, for example, full density would give you a solid 'spray' of paint as you moved along. on 50% density you'd get a more speckled effect. pshop may call it something else, but i'm sure they have it.
so, if you were using the airbrush on a 5% density and a light opacity, you wouldnt be getting a lot of 'paint' coming out at one time. it would be very light and pretty speckled. so, in order to get a tone or texture you want, it's a good idea to start light and simply add more passes with your brush, or, build it up. i find this especially true when colorizing skin. the biggest tendency is to try and put it all there at once, and this rarely works very well.
as for what colors to use, there is a skin color chart floating around here on teh forums somewhere. i dont use it currently. i tend to go more by the picture and what's already there, but others swear by it. sorry, dont know the exact location of where this chart is currently.