Start by adding your image into Gimp. You can do this by either dragging the image in from your files or clicking on File then clicking on Open and then selecting your image.
Once you have done that, right click on the image in the layers section.
Click the top layer and go into Colours at the bar at the top then select Hue saturation. Turn the Saturation all the way down.
Then right click on the top image and select Alpha channel. Once you have done that, erase everything from the top layer that you want to be coloured.