The materials out there for this particular demand of coloring are pastels - powdered pigment bound together in different ways. They vary in form but are essentially the same in substance. The ones that can be used both as wet and dry medium are chalk pastel, soft pastel, pencil pastel, and pastel in pans.
For flawless skin coloring in particular my personal bias is for PanPastel. Two months ago I walked into National Bookstore bent on paying for three pans when, lo and behold, I was told that the price dropped just the night before and all of a sudden my budget (and a couple more hundreds) could afford me five pans instead! Whooopeee! (I believe the distributor already settled on the lower price point since up to now the new stocks I've seen remain marked with the lower price.)
The dust is gorgeous because, for one, it easily clings to paper unlike powdered Mungyo soft pastel or Mungyo chalk pastel. I only need to spend half the time moving the dust around before the color is absorbed by the paper's surface. Another reason I like PanPastel is because for skin coloring you'd want to visually achieve perfect smoothness which fine dust coloring gives. You eliminate pencil strokes from your work. If you take extra time to choose your application tools like eye shadow brushes, foam tips, spotter painting brushes (got mine from Craft Carrot) this powdered pastel is very easy and fun to apply and gives maximum satisfaction (for me at least). Also I find that I can also afford to make mistakes since the dust is erasable. Very good erasers I've used for this are kneaded rubber ones and the fancier but effective Derwent electric eraser.
The biggest limiting factor in choosing PanPastel is the hefty price. With one pan you can already buy a set of other coloring materials with varying colors. But there's a way around that. I found myself suggesting to colorist friends to split among them a pan and put them in those tiny containers one finds in BEABI stores. Since the PanPastel dust is a very efficient medium you really only need a small amount each time to spread around and blend. Believe me a little will go a long way.
For me another good pastel product to use, again for skin coloring in particular, is pastel pencils. The one I'm using is Stabilo's CarbOthello. The set of 60's has a good range of browns that you can use to tone. But don't overdo the browns so you don't end up with a darker complexion that what you want to achieve. Again, these are pencils so if you want to eliminate lines you'd have to go ultra light with your strokes. Good thing going for you is that the dust is a joy to spread. Limit your blending to the edges though otherwise you'll find yourself reapplying endlessly because you keep rubbing dust off ever time your tools touch the medium.
Someone asked me how Derwent Pastel Pencils compare with Stabilo's CarbOthello. My colorist friend, Anwy, made me try them side by side and immediately I could tell that CarbOthello's colors are more striking, and smoother to apply. It appears that the pigments of CarbOthello are more fine.
What's the advantage of that? Like many colorists today the choice of medium is essential in enjoying our craft. Stress-free coloring nga e. So generally I understand why many shy away from the less vibrant, or grainy brands that makes it harder for us to achieve satisfaction.
I have to mention too that CarbOthello pencils have good- to high- lightfastness rating, not bad at all especially if you're particular about the selling quality of your end product.
Oh wait, here's the fixative I use by the way. Thanks Mark Dean Lim for suggesting this to me. When applying stay outdoors or in a well-ventilated area. I have yet to try in a coloring book so I can't say anything about that, but if you will protect the rest of the book by putting a big board or paper beneath the page you're spraying on. And finally, since it has binding agents it's best you don't touch your work while the material is still wet. Wait for about an hour for everything to adhere together and for the solvent to dry.
***Second colored image from Mystery Women by Och Umlas