Dear 100 Hour Board,
Regarding Board Question Board Question #49921:
Photoshop Express (https://www.photoshop.com/express/landing.html) has lots of fun and free features including one called color pop! This is a good tool, but sometimes it can pick up on other color sources that you don't want which can be a pain.
Another choice is Picnik, which can be found through www.picnik.com or used with your Flickr account. If you go to the Create section, add the Black and White effect, and then use the Paintbrush to erase the black and white and bring back the original color. This is really useful for controlling the colors you want showing.
Have fun editing!