… is a nice little website application that I found, when trying to figure out how to isolate a single colour in a photo.
It’s pretty easy to use picnik, though it is a bit slow. Upload your photo (don’t even need to register) and it will take you to the ‘Edit’ tab. Switch to the ‘Create’ tab instead and select Effects. There is a basic effect called Black & White which lets you “paint” over the sections of the photo you want in black and white (or in colour). Good for people like me who don’t have software like photoshop to do it with (and who can’t figure out how to do it on graphic converter).