When the image that comes out of the camera in black and white, all you have is black and white. All the color information has been lost. You can't tell a red from a green or blue. Why do you care if it's a black and white? If you plan on doing any editing of the photo, you do care about the colors. Here's an example to illustrate my point.
Here's a color image shown in Lightroom 4
Although it was still winter when I took this, there are some colors here. Blue sky, red bricks, green roof, and just a little green grass. Now, here's the same image converted to black and white in Lightroom.
By default Lightroom will create a black and white image that maximizes the distribution of grey tones. Notice how the color channel sliders on the right have been automatically set by Lightroom. Most of the time I use this as a good starting point. In this case, I want to make this photo a bit more dramatic.
I start by making the sky more dramatic by increasing the contrast between the dark (blue) areas and the light clouds. Because the color information is still there in the photo I can easily do this by lowering the blue channel by grabbing that slider and pulling it to the left.
Notice the difference in the sky. If I pulled the slider all the way to the left the sky would be almost black, which may be dramatic but it does not feel natural.
Second I want to lighten the front of the Salt House. I do that by adjusting the Red, Orange and Yellow sliders until I get the effect I want. I also want to darken the roof a bit. Because the roof is green I can easily do that by pulling the green slider to the left.
Sometimes the color I want to adjust is not a pure color but a mixture of several colors. Lightroom has a cool tool for adjusting specific colors in an image. At the top left of the B&W panel on the right is a small target. Clicking on the target activates the target adjustment. Put your cursor in the place in the image that contains the color you want to adjust, left click and drag the mouse pointer up to brighten that color or down to darken it. When you do this you will see more than one slider move and some sliders will move more than others. Click Done to turn off the targeted adjustment mode.
In this example the locomotive is a bit dark and some details cannot be seen. I'd like to lighten it up a bit. Because black is the absence of color you can not use the color channels to make this adjustment. The same is true for white, which is all colors blended together. The best way to adjust these is to adjust the region of the tone curve using the tone curve or tone sliders or to use the adjustment brush.
If I was working with a straight black and white image right out of the camera with no color information it would be difficult to impossible to make these adjustments. That's why you should always shoot in color and then convert to black and white later on the computer.
I used Lightroom for this example but it works the same in other photo editing packages, including Photoshop and Nik Silver Efex (now part of Google).
Remember, don't take short cuts when you capture an image. It may sound like a good time saving step but you may not be able to get the end result you want if you short cut. The same rule applies for good composition, exposure, and technique when taking a photo. Get it right in camera without short cuts.
