The way the adjustment layer works is by mapping colors to grayscale luminance values. Put simply, you can use the color sliders in the Properties palette to tell Photoshop how bright or dark you want a certain color to be represented in grays. For instance, drag the red slider down (to the left) and watch as areas that were red in the color image turn into dark gray or even black pixels in the converted image. Drag the blue slider up (to the right) and watch as blue tones from the original turn into light gray and white values in the black and white image.
The wonderful thing about this flexibility of tonal control is that it’s a great way to dramatically change the look—and perhaps more important, the “feel”—of a black and white image. With the right combination of sliders, you can make a black and white image light and airy or dark and brooding, even though they both started off as the same color image.
But what happens if most of an image looks good with one combination of sliders, but some part of that image looks better with entirely different slider settings? This happens, commonly, with portraits like the example here, where too much movement of the red, orange or yellow sliders has a negative impact on the face. In this specific example, I liked how bringing down the red slider gave much of the image a dark, low-key vibe. But I didn’t like how it exaggerated freckles and made the lips overly dramatic. For the face, a much more appealing combination of sliders rendered red values much brighter and minimized freckles and any facial redness, but it also sucked all the drama out of the rest of the scene. The ideal look would be a combination of the two adjustment layers: one for the face, one for the rest of the scene.
If you just did one of the black and white adjustment layers and then used the paintbrush on a layer mask to selectively minimize the strength of the black and white conversion, you would reveal the color layer below and it would look, quite frankly, odd. (See the very odd example below.) But if you did one black and white conversion via an adjustment layer—say, the one with dark red tones—and then another black and white adjustment layer on top of that, only the lower of the two layers would impact the conversion—unless you use a layer mask to paint away elements from either layer to let the second black and white adjustment layer show through where you created the layer mask.
In practice, as shown in the example here, the first adjustment layer was the dark red layer. The second adjustment layer, which produced the more appealing conversion in the face and neck, was on top. I then used a large (almost as big as the face), fairly soft paintbrush to paint on the lower layer’s mask to eliminate the “dark red” conversion from the face and neck, letting the top adjustment layer work its magic there and only there.
If you needed to be more precise than a freehand painting approach, you can always make selections using data from any of the layers and then paint that selection away on the adjustment layer mask to produce an even more precise change to the black and white conversion. There’s no limit to how you make the selections or paint on the mask: whatever works is right.
I prefer this approach for combining black and white conversions because it’s the simplest and most intuitive to me, it works like dodging and burning in the traditional darkroom, and I can always come back to refine any of the stages of the conversion—from the particulars of the mask to a given layer’s opacity or the specific slider settings on either of the black and white adjustment layers. It’s a really simple technique, but it provides lots of power and extreme control over the specifics of the black and white conversion.