Photo Black & Color is a portable tool that allows you to make a partial de-saturation (as shown the screenshot below) on a picture by selecting the colors you want to preserve.
The partial desaturation is the fact of protecting a colored subject and of leaving the rest of the image grayscaled.
To get good results, better choose images with one main subject with colors different from the background colors, such as flowers, fruit or other.
The application is very simple to use. The included help file lets you do the things very easily. You can free download (900Kb) it at source.