Paul, fading everything but a specific selection to black and white is a fairly simple process, but you might want to give it a little practice. I'll show you the steps I use to accomplish the task using the Gimp photo editing software (free download if you don't already have it).
Originally Posted by Summer Sun
Step 1: Open a photo of your choice. This effect looks best on photos that focus on objects that will contrast their backgrounds normally (for example a single red rose amidst a field of greenery). In this example, I'm using the red hero car from LMA. It would be ideal if it were centered in the shot, but it was just the first picture I opened, and well... I'm too lazy to go hunting for another one.
Step 2: Once you've got your photo open, select the free-hand select option from the toolbar (select hand-drawn regions). It's highlighted in figure 1 below.
Step 3: This is the hardest part of the whole process. Zoom in as you see fit using the percentage number in the bottom left of your screen. Typically the closer you get, the more preceise you can be with your mouse. Holding down your left mouse button, draw a perimeter around the object you want to accent (figure 2).
Note: If your object has gaps in it that you would also like to fade out to black and white (such as -using aeronautxearth's example- the area between Mickey and Minnie's shoulders and noses), after you have selected the perimeter, release the mouse button, hold shift (to deselect), and then trace the perimeter of the gap you would like to deselect. If this is your first time doing this however, I recommend sticking with objects that lack these gaps. Practice on something easy like a solid shape.
Step 4: Once you have selected the object as you want it, go up to the select button on the top toolbar, and from the drop-down menu choose "Invert" (figure 3). What this does is flips your selection so that what was selected previously is no longer, and what wasn't... well, now it is!
Step 5: Now that the background is selected and your soon-to-be highlighted object is not, use the Tools button and navigate the drop-down menu to Tools>Color Tools> Hue-Saturation (figure 4). Using the toolbox that opens up, select the "Saturation" slider (the bottom one) and move it all the way to the left to completely desaturate anything you have selected. If you have done everything correctly up to this point, your photo should now have no color, with the exception of what you wanted to remain (see figure 5).
Step 6: Now, assuming there are no more changes you wish to make to your photo, simply go back up to the selection button on the top and select "None" (figure 6), which will deselect everything.
Your finished result should look something like this:
Now, this was a quick tutorial, done in just a few minutes using the first picture I could find. If you would like an even greater in-depth explanation and maybe another example, just let me know and I'll do what I can to help.
Oh, and PS: I think this picture would look incredibly cool if I left the flames in color too, but that would be VERY difficult to pull off while making this instruction set. Maybe I'll tinker with that a little later on my own. ;-)