There are a few ways to do selective coloring.
What method is best to use depends on your skill level with photoshop.
Personally, I like to use layer masking because it's non-destructive.
There's also the simple method of selecting the area/s you want to retain color in and copying them to a new layer. Then you apply an adjustment layer above the original layer to strip out it's color.
That's a rather archaic method though.
I'll explain the masking method to you.
Make a copy of your original background layer, then click off the visibility of the original background layer and select the background copy.
Add a channel mixer adjustment layer, and put a check in the box that says 'monochrome'.
On the right of the new adjustment layer, click on the layer mask icon to make it active.
Go into the channels tab of the layers palette and click on the layer mask channel on the bottom.
Now select your paintbrush tool and choose a brush suitable for the size of what you're painting out.
Now use the paintbrush tool with transparent red spray paint to fill in the area where you want to retain color.
You'll want to zoom in and out alot to paint it with accuracy.
Now click off the layer mask channel and go back into the layers tab.
That's it, you can tweak some of the settings all you want but that's the basic method.
I'm pretty sure I didn't leave anything out.
Good luck.