If the shot is already black in white (was never in color) this could be quite hard. But otherwise its possible. Just beware that this can be time consuming.
If you're lucky, the color that you want to stand out will be the only instance of that color in the shot (lets take red for example). If you have a red book, and there is no other red in the shot, then you are in luck (rarely the case). If luck is on your side (only the book is red), then you click on filters, and add the
secondary color corrector filter. basically you have to sample the red color, and then desaturate the rest of the image. There are many good little tutorials on youtube that you can find easily if you search something like "sony vegas sin city"
If you're unlucky, red will appear elsewhere in the shot (almost all cases). For this, duplicate your video layer. The bottom one will be completely black and white. The top one you have to use the same method as described in the above paragraph, and then mask out the object that you want colored. Again, you should be able to find plenty of youtube tutorials on masking in sony vegas.
As I mentioned this is really time consuming. I did this technique for the short I cut for the Batman Consecution project. The video was only 6 minutes long, but it took me many many hours to color and mask. Be prepared for some time commitment. Here's what I came up with: