You can use the black and white adjustment directly on the image layer by going to Image > Adjustments > Black & White (or alt + shift + ctrl + B) or you can make an adjustment layer by going to Layer > New Adjustment Layer > Black & White. Both will allow you to change how black or white each primary color (reds, yellows, greens, cyans, blues, magentas) appears in the image and add a tint to make the image duotone. If you use the black and white adjustment directly on the image layer that's all you get. Once you hit "OK" you have to undo it and reapply the adjustment. By using an adjustment layer you can alleviate this issue.
When you create a black and white adjustment layer it will add a new layer with a layer mask on it. Having this extra layer opens up several opportunities that you can't get by using a direct image adjustment. You can now come back to the layer and adjust the color sliders and tint as often as you want. You can also create a clipping mask, use blend modes, or create a gradient on the layer.
Here are a few images that I have created using black and white adjustment layers:
For this image I used the blending mode Darker Color. When using this blend mode, adjusting the color sliders determines what colors come through and what colors are grayscale, as well as how dark the grayscale versions of those colors are.
This one was created by using a radial gradient that becomes transparent in the center on the black and white adjustment layer. You can make this by using the Gradient Tool (or hitting G) and selecting Radial Gradient. Note that the gradient needs to be black to white for this to work. The black areas will be the transparent portion of the gradient and the white will look normal.
This one was created by using a clipping mask and the Darker Color blending mode. I cut the car out of the image, used it as a clipping mask, and set a black and white adjustment to Darker Color on it so that only the car would have any color. The background just has a simple black and white adjustment layer.
On this image I also used the Darker Color blending mode. I set it so that only magenta and reds would show.
For this one I set the blending mode to Linear Light and set all colors except Reds to -200 on the black and white adjustment layer.
As you can see there are many creative uses of the black and white adjustment layer. If you have any questions or suggestions about anything in this post, or in general, feel free to ask. Also feel free to post any images you've created in the comments. I'd love to see what other people are doing with adjustment layers.
