i love this style of art, making a picture grayscale then making one color or object stand out by turning it back into color.
so i have handy way of doing this, there are several ways of doing it, i havmt found a new way or anything but i thought id share it with you to save you looking elsewhere, so....
1. load you image into Photoshop(make a duplicate if you wish, just encase you screw it up).
2. at the bottom of the layers tab hit the button that looks like a black and white circle (opens up the adjustment Layer option).
3. click hue/saturation and turn the saturation down to greyscale.
4. click on the mask for this new layer and use the erase tool to paint the color back, and the brush tool to paint the greyscale back.
5. there simple as that.
5. Tips, use the spacebar to move around the image, and the brackets buttons(below the + on your keyboard) to change the brush size, right click gets you the brush menu when using the eraser and the brush options.