Mixing Color & Black and White
Oddly enough, our technique begins with a color image, a Fujichrome Velvia slide that was scanned.
The first step is to Desaturate, or eliminate the color from the image while keeping it as an RGB file. The reason we keep it as a color file is that we are going to use the colors it contains later. My favorite way of converting to black and white, using Photoshop, is to work with a tool known as the channel mixer.
You find this tool at Image>Adjust>Channel Mixer. But before you do this we're going to create a duplicate layer, a copy of the original that sits on its own layer. The reason we do this is so that we can play with that layer independently without affecting the original. It also gives us access to working with a Layer Mask, which we'll use to "cut back" from our channel mixer monochrome conversion.
So, first go to Layer>Duplicate Layer and name the layer whatever you like; here we've called it "monochrome."
Now look back at the Channel Mixer dialog box. We open this tool at Image>Adjust>Channel Mixer. When you open this dialog box be sure to check the Monochrome box in the lower left. Now move the sliders until you like the look of the black and white picture. The green channel affects the brightness while the red and blue channels handle contrast. Don't make the image too contrasty, as this will "burn up" the white highlight in the clouds.
Here's how the picture looks after the Channel Mix was applied.
Now we want to bring some color back. To do that we want to add a layer mask to the duplicate layer. The mask allows us to cut back, if you will, to the layer underneath it, which in this case holds the color information. To add a layer mask make sure that the duplicate layer is highlighted in blue, which means it is the "active" layer, the one that will be affected by any actions we take. Go to the layer palette and look for the symbol that looks like a circle inside a rectangle; click on it, and you'll see that symbol in the background layer strip to the left and a blank rectangle on the right.
Next go to the tool bar and select a paintbrush. Make it small enough to be able to work on details. You can change the size of the brush using the Brush palette or simply by using the bracket keys on your keyboard--the right bracket key makes it larger and the left bracket key makes it smaller.
Now just paint back the areas of black and white that you want to change back to color. Here I painted back a strip in the sky just above the bright highlight and a good deal of the blue water. When I painted I went to the top toolbar and made the "exposure" of the brush about 50%; this gives more control when painting a mask, but keep in mind that each stroke has a cumulative effect.
Here's what the image looks like after the mask application.
I liked the effect but wanted the foam on the waves to be brighter. This kind of nuanced effect can be done at any point within your work, but it's good to get the main work done first and then make judgment calls for "tweaks" later.
To do this I went back to the Layer palette and highlighted the background layer, the original image. I then went to the toolbar and chose the dodge tool.
I went to the top toolbar to adjust the strength of the dodge tool. I chose "midtones" at 30%. Again, modifying the tool like this gives you more control.
I adjusted the brush size as above (with the bracket keys) and went to work making the waves and some of the spectral highlights in the water lighter.
Here's the final image after dodging.