Requested by kirtash_girl at Ask the Maker 3.0 (my thread). Hopefully this is what you were looking for! :)
This is a quick guide on how I do black and white icons. I talk a little bit about cap selection (the whys and hows of the caps I pick), my general process for black and white icons, what textures I tend towards when I'm doing a b&w icon, and some random tips and tricks/notes. This is by no means a complete guide, and there's definitely not a 'right' way to do B&W icons (this is just how I happen to think about it).
If you have any questions, please let me know! I'd be more than happy to answer them. :)
When it comes to picking screencaps for b&w icons, I tend towards two different types of screencaps: simple screencaps, or screencaps where I can remove the background so that my subject won't get lost in it.
B&W icons work best when it is easy to find and focus on the subject of the icon; without colour to pull the subject out from the background, subjects can get lost if the icon is too busy or crowded. Thus, simple caps work best. What constitutes a simple cap?
(Click for full size.)
I'd consider all three of these "simple" caps because the camera has either closely cropped the subject for me, the subject is positioned so that a close crop would be good, or the background of the screencap is relatively plain (and therefore won't distract from the subject).
The other alternative, if you have a cap you think would make a spectacular B&W icon but for that busy background, is to cut your subject away from the background. Some good tutorials to look at for this can be found at good_tutorial (under this tag). The plus side of removing the background for an icon that you know will be b&w is that blending the new background and your subject together is easier, because your colours don't have to match! :)
I'm a sucker for a lot of contrast in B&W icons. While more matte tones work for some icons, I still tend towards really dark blacks and light whites for my icons because I like the aesthetic. While the shadows can be achieved through a combination of curves and/or levels, soft light layers, and brightness/contrast adjustment layers, I've found that if you start with a cap that already has the shadows there, the icon will come out much better. For instance:
(Click for full size.)
The first thing I do when I start iconning (b&w or colour) is crop my image, because it's probably my least favourite part and I don't want to spend a ton of time colouring an icon only to find that there isn't a crop that I like. Usually, I just paste my image onto a 100x100 canvas and free transform until I'm happy with its placement (when I worked in PSP, I just used the crop tool, but I found it easier to work with than PS's crop tool).
For b&w, the main thing I've found to keep in mind is make sure the focus stays on your subject. Close crops where there are lots of shadows, centered negative space crops, etc. Without colour, it's easy to lose your subject if your background or textures are busy.
I pretty much have two kinds of black and white icons: the ones where I sit down and plan for them to be b&w, and the ones where they end up b&w after being coloured normally. For each 'type' of colouring I'll give an example, to show how the process works.
For icons that I know are going to be b&w, the first thing I do is put down a black and white gradient map (found here in CS5), followed either by two curves layers or several screen/soft light duplicates of the base, depending on what looks better. If I use curves, I use the first layer to brighten the icon up and the second to introduce contrast back into the icon (usually, this second curve is 'S' shaped). From there, everything I do is strictly to increase the brightness and/or contrast as apporpriate - which I do using curves, levels, and occassionally a brightness/contrast layer. This obviously differs depending on the image.
As an example, for a simple icon that I intended to be black and white, this was my process:
The other b&w icons I do are as often happy accidents as attempts to make an icon specifically b&w, lol. Either the icon looks as good or better in b&w as it does in colour, or my attempts at colouring an image are either overwhelming the subject or just plain not good, and so they end up b&w instead. For instance, these icons are all b&w, but had colour versions already:
My process for getting these b&w isn't so different from my regular process. Once I've finished colouring the image, sometimes I'll check it with a b&w gradient. If I like what I see, I might save the colour version and then make the b&w alt. by tweaking the contrast and/or textures as necessary. For instance:
These are approximations of the settings that I tend to use when making an image b&w. Obviously, it depends on the icon, and in all cases my main goal is really just to increase the brightness and contrast of the image. Usually, I'll use these tools in combination, not individually.
For more about how these tools actually work, see this guide to colour normalization that I wrote a while back, or browse through some of the tutorials I listed at the bottom of that post. :)
Layer blend modes.
I use lots of soft light layers, with screen layers underneath to keep things bright. I find the contrast gets too high if the layer is set to hard light, but if I use multiple soft light layers at various opacities, I've got better control over how much contrast an image will end up with. Soft light darkens the shadows beautifully.
Depending on your icon, textures might make all the difference in making your icon look like a finished product. These are some textures that I find myself coming back to again and again for my b&w icons, and what blend mode I usually use them on. Essentially, the trick is to make sure the texture complements the subject, not overwhelms it, because otherwise your subject might just fade into the background or get lost in the texture.
Below I've given some examples of different types of textures that I like to use, and how they look on an icon. I should note, though, that I'm big on using a whole ton of textures for colouring and lighting, so I've only listed the most obvious texture, not necessarily all of the textures on that particular icon.
All of these textures are by lookslikerain @ soaked
screen | soft + hard light | screen | screen
hard light | normal (background removed) | soft light | soft light
midnight_road | midnight_road | soaked | lumsx
"Complex" B&W. (aka duplication)
Pretty much throughout this entire guide, I've been emphasising simple composition to make sure that the subject doesn't get lost in b&w. Obviously, I'm far from an expert, and all of this is pretty much just what I've concluded from my own experiments. And, as such, there are always exceptions to things that I've said. For instance, sometimes b&w can work with complex icons better than colour would:
In both of these cases, I've just duplicated the same base image - but, if these icons were colour, the colours would completely overwhelm the icon. The original icons are the black and white ones.
So, these aren't really textures or gradients, which is why I'm not mentioning these up a little bit higher. Lighting is really important in a black and white icon because it is really what makes an icon stand out. Usually, if I'm not happy with how the lighting in an icon looks, I'll create a new layer, set it to soft light, and paint over various areas with a soft brush to brighten or darken those spots (light gray to brighten, dark gray to darken).
(The "light blobs" layer is actually set to soft light, 88%. I painted over the blank layer with a largish brush (between 20 and 35 px, depending on the section), as soft as it could be.)
These are some other resources that you might find handy when you're looking at b&w:
Tips & Tricks for Black and White by justmyb0nes
A Guide to Lighting by arctic_flower (colour icons, but the ideas can be used in b&w too)
good_tutorial has several icon tutorials that feature black and white that might be helpful (tag).