If you still have a colour copy of the photo, this is easy with almost any photo editing software (gimp, photoshop express, and many others). If you saved the B&W over the only colour copy, you can do it, but it's 50x the work (you have to paint the colours back in manually). There are many different ways to get the look.
1) Create two layers, top layer is b&W version of colour image on bottom layer. Black out layer mask to let colour shine through.
2)Colour image in 1 layer, adjustment layer to B&W on top, black out layer mask to let colour through.
There are many more ways, this is just something that can get you started.
BTW, you may get flamed for this approach (selective colorizing). Many people regard it as tacky and played out at this point. If you like it, go ahead and use it, just try not to overdo it.