Open the full-color version of the image you wish to transform.
Select Visible.
Layer -> New Layer. Set it to Transparency.
Edit -> Paste Into.
Anchor the pasted image onto the new Layer.
Colors -> Desaturate.
Basically what you’ve done is created a Black and White version of the image on a clear transparency above the color version.
Then use the eraser feature to erase the part that you want colored. Zoom in, use different size brushes, etc.
This results in a really cool effect where you have the part of the image that you ‘erased’ in full color, and the rest of the image in black and white.
If you want to, try applying a Gaussian blur to the layer with the black and white. This makes the black and white ‘background’ appear blurred, while the main color image stays focused.