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.
look into how airbrushers paint. They use this technique all the time, but use a black background to start with the paint everything in whites, no greys or blacks, youll get the tones from not using so much white on areas that you want darker, letting the black background show through creating your shadow...its really hard to explain but watch airbrush stuff on youtube like this. Will give you a btter idea of how to go about it, when trying to apply colors over it. Its a very very usefull technique and i love it.
heres one showing how to go about painting the image in white 1st....[link]
doing it digital is basically the same thing youll just have to figure out what layer option is better,color,overlay,hard/soft light. some give you different effects, or even painting over it with light opacity and building up the colors some, youll just have to mess around with it to find what works best for what your trying to go for.
if i do a portraits of people i dont do it this way digitally, but everything else i tend to use this technique in there somewhere.
You can call this an underpainting, and then the colors are glazing if you want traditional terms.
One thing though, is to never worry because your in digital. You can just make another layer for color and color it over. If you don't like it, you can use adjustments, delete it, paint over and keep going.
Just experiment with layer styles until you find one that clicks. The most popular are multiply, color and overlay. There is no sure fire way to go from black and white to color. Some just do a quick color layer then add all the finer details in color. Here is Dave Rapoza going from a monochromatic image to color. [link]
And to answer your question specifically if it's too dark or not. What you have is fine. I think you'll notice quickly how each layer style will effect the image.
the levels tool and histogram will tell you if your picture has a high or low contrast, the information panel also helps. Usually you don't want any main element with less than 20% brightness or over 95%, below 10% brightness everything looks basically black. All that makes more sense when printing though, but you can also apply it to on-screen stuff.