Not bad!
BTW, if you want to keep the shadings make use of the layers in Gimp.
Activate the select tool and click into the white background area of the page. Invert the selection. This will give you a quick and easy selection around the parts outline:
Create a new transparent layer in the layer dialogue box:
Choose your colour and bucket fill the selection or paint the desired parts with the paintbrush on the new layer:
Play around with the different layer modes and transparency settings till you have found a pleasing result. This is using "colour":
soft edges:
burn:
Since you are working on a new layer you may manipulate the painted areas further with the eraser and other tools without altering the base image. Then save as *.xcf file to keep all layers intact for future editing and export as *.png to flatten the image for printing.