if you are familiar with Adobe Photoshop (I use CS2), create a new image with the same dimensions of the original.

Drag and drop the original image as a new layer;

make the entire image Grayscale;

Make the image back RGB (the grayscale will be preserved);

Now drag, once again, the original image (in color) to a new layer;

You pretty much created an image with 2 layers: one black and white and another, in color, on top of that;

Get the erase tool and nice and slow (I use a Wacom tablet) delete everything you don't want in color. The back and white layer will still be preserved;

Once you are done, flatten all layers and save as JPG.

With practice it takes about 5 minutes to do a simple image like this one.

Hope this helps.

R.