Layers>Duplicate or right click in the layer palette and duplicate.
On the background copy layer, you want to desaturate it, make it black and white. You can do this from Adjust>Hue & Saturation>Hue/Saturation/Lightness. Pull the saturation slider to 0, the other sliders should be zero as well.
Now do Layer>New Mask Layer>Show All, and then choose the Paint Brush with white as the
fill and background color and Black in the foreground/stroke color box.
With black, paint on the layer mask where you want the color to show, if you make a mistake you can switch the color to white and fix it, just repaint with white.
When you're happy with it, flatten the layers(Layers>Merge>Merge All) and save.