These tones come from the lightness values of the gradient colors as they're defined in the Lab color space, which is modeled on the way humans perceive brightness. Our eyes are most sensitive to greens and yellows, so those colors are rendered lightest. We're much less sensitive to blue and red, so those colors come with a lower L value in Lab and are rendered darker.
Figure 3: This shows the sRGB gradient colors plotted in the Lab coordinate space in Color Think. The "L" scale for Lightness is the vertical axis in this coordinate system, so colors with higher lightness values appear higher in this space and those with lower lightness values, such as the blues and reds, appear lower.
By using the L values in the standard grayscale conversion, Photoshop uses a model that's based on how our eyes actually respond to the brightness of different colors. But not all of the processes choose brightness values in the same way, and it's the brightness values that determine color contrast.
Figure 4: Note that in the top view, the gradient colors follow the outer boundary of the sRGB color space, where they're at their maximum saturation.
Figure 5: Here's what happens to the same pure colors when they're simply desaturated using the Hue/Saturation dialog in Photoshop. Because Hue/Saturation works in the HSL color model rather than in Lab color, the very saturated colors in our gradient are all rendered as middle gray.
Beyond the standard grayscale conversion and desaturating in Hue/Saturation, there are countless other techniques for creating black-and-white in Photoshop. On top of that, there are countless plug-ins out there, each claiming to possess the magic elixir that will make your black-and-white conversions "unique." But by far, the most important aspect of making good black-and-white representations is color contrast. And the raw controls that we now have in Lightroom and Adobe Camera Raw make creating great color contrast a piece of cake.
ExamplesLet's take a look at two very different examples of this. Figure 6: First, in this photo of the restaurant front, we have an image that contains colors in nearly every one of Lightroom's color luminance ranges.
There are two starting points for black-and-white in Lightroom's HSL tab. Figure 7: When you first convert to black-and-white, the default behavior gives you the Auto mix. The Auto mix generally darkens the greens and yellows and lightens the blues and purples, but the actual mix that you get will be different with every photo. Auto always tries to give you the best color contrast that it can.
Each time I start a black-and-white conversion, I always try both of these options. But, as I said, these are just starting points. The real work (and fun!) comes with working your way down the sliders, using your eyes—and your brain—to create a mix of your own. And it's easy. The reason why I love these controls is because using them becomes a completely visual exercise. There are no rules, there are no best settings—just your eyes and the sliders. Figure 8: Start at the top, push the Red slider back and forth while you're watching the image, and find a position for it that makes your picture look the best. Visually. Then move to the Orange slider and repeat until you get to the Magenta control at the bottom.