The point of drawing in gray-scale is to get an eye for value. At first you can make the reference black and white too, than after some practice you can probably paint from a colored reference.
If you get the values down right it's easy to put color on top using different layer blending modes. I think I have a few tutorials saved about it in my tutorials folder in my favorites if you want to search through there. I found tutorials on this technique are hard to find since it doesn't have one common name. The thing is it's not easy to get the values right, and I often find using layer modes to add colors sort of dulls the image.
As for your painting, it's pretty good for a first try in B&W. The skin is just a tad bit too dark/glossy looking. Tone it down a bit and you should be good.
There aren't any values you should avoid. If something would be white, it's white, if it's black, it's black. Of course, sometimes you make things extremes that aren't extremes, and that's where problems come in. You have to develop a good eye.
Thanks. I did want a glowy skin effect, but I agree that the gradations might be a bit too drastic. I'll have to work on making next grayscale softer: better grayscale, fewer things to fix during coloration process.
I think actually lightning around the skin with a soft brush and lighter skin might actually make it seem light it's glowing more. Also don't forget if we're talking about real light coming from her skin it would affect things around her and case a light on her hair, etc.
Doing a grayscale first and then adding color does seem like a lot of extra work to me, but I figure if nothing else, if you can't get the color right, at least you still have a good grayscale painting to show for it. Also doesn't seem like it would look right with colors laid over the grays, but I'm going to try it and see how it looks.
I've been looking into doing grayscale paintings lately, too, and adding color later. Doing just a grayscale is wayyyy easier than color, but I imagine adding color will be a bit difficult. The first one I did I had the same issue with making it too dark initially, and ended up having to correct that later. Just keep in mind complete white or black are used very judiciously, and I would even say rarely. I always try to establish my extremes (lightest lights and darkest darks) first, which gives the whole thing some parameters, then I go from there. You've probably noticed that lower contrast pictures are more difficult to do and take more skill.