First open the image you want to edit in GIMP. (File>Open)
In the Layers Dialogue Toolbox on the right side of the screen, right click the picture layer and click “Duplicate Layer.” (What this does is create a copy of the image so one copy can be grey and the other copy is colored.)
Pick one of the greyscale options and click Okay.
You should now have 2 versions of your picture, a grey version and a colored version. Make sure the grey layer is on top of the colored layer in the Layers window on the right.
Now go the “Layer” menu at the top of the page, “Transparency” and “Add Alpha Channel” (This makes an invisible layer for you to work with so when you erase the grey parts, it wont be white underneath)
Boo-ya! You should now be able to erase the grey version of your picture to reveal the colored version underneath! Use the eraser tool in the toolbox on the left hand side.