Here's how I've been handling the greyscale shading lately, in a step-by-step format. I'll use panel six of this installment
as a reference. All color values will be in RGB format. I use PSP 8, so you may need to find equivalent tools/commands in your program for this to make sense.
NOTE: before you do anything else (except for possibly erasing background bits you aren't going to keep), promote the image to 24-bit color.
Now, begin by checking the RGB values of the image sections that require color. Break the image down into sections according to the base colors you intend to use (flesh tone, hair color (purple here), clothing, mouth, etc.).
If a section has a noticable instance of pure white (255/255/255), then don't change anything. For example, fTedd's skin in the above installment has large areas of pure white, so for those I make no adjustments. If a section does not have a significant area of pure white, you'll need to make some adjustments. fTedd's hair is a good example of this - it has multiple grey shades, but no visible white.
Start by selecting a single section you wish to color on the base/background layer (don't create other layers until later). If you have a magic wand tool, you can start with that, but you may need to hand-select (or hand-deselect) things at times. Make sure you stay inside the boundaries. Over time, you'll be able to tell by eye how close to the boundaries you can come, but for starters try to select as many pixels as you can up to the pure black (0/0/0) outline.
It's a good idea to save at this point, and perhaps save the selection to the alpha channel as well (just for good measure, in case you need an emergency recovery file).
Once your section is selected, adjust the RGB values so that the lightest pixels are converted to pure white. In PSP 8, you can do this by using the Adjust | Brightness and Contrast | Levels dialog, with the channel set to RGB. This lets you define a minimum and maximum input level (with an adjustable median), which you can then map against a desired output scale. For fTedd's hair, the lightest color is 120/120/120, so I plug 120 into the maximum input level number (all three values will be mapped identically). I leave the minimum set to zero and the median set at the default. I map the output to a scale of 0-255, which means that anything with a value of 120 will be changed to 255, and so on down the scale. It's a purely mathematical function, but it's a lot faster than replacing things by hand.
Your end result will look like this:
Note that I did not select any of the hair that was already pure black (behind the neck, shoulders, etc.). You don't need to color this, so don't waste time selecting it. Just pick out the grey values.
Once you've adjusted one section, move on to the next. (BTW, There aren't any other sections that need adjusting in the example - just the hair.) When you've finished adjusting all of the sections, save a copy as a baseline reference and then start applying colors to layers on top of the background (not to the background itself). Set the layers to a Multiply blend setting at first - that's usually the only setting you'll need for colorizing people, but be ready to improvise if things don't look right.
You can apply this technique to backgrounds (i.e., non-people) as well, but most of the time it's not required - there's usually some white to be found somewhere in the background. Just keep in mind that the technique is available if you ever do need it.
I hope I haven't left out anything (curse bedtime and going past it for the third straight day), and I hope at least some of this helps. If anything doesn't make sense (possible - bedtime, remember?), please don't hesitate to ask questions.