I found this tutorial (http://youtu.be/d_OudUWhfNo) on how to use the G'MIC "Colourize (comics)" filter to do lineart colouring in GIMP - here's a "worked example" featuring a current art project. This technique not only works with digital inking, but also with digital pencils - and avoids all that tedious mucking about with area selection or manually colouring between the lines.

Before you begin:
* You have GIMP and G'MIC installed on your computer. (Both are available for Mac, Windows and Linix - I'll be showing the Linux version)
* You have existing lineart opened in GIMP.

Steps:
1. Create a transparent layer over your lineart (Coloured Spots)
2. In each area you want to colour, place a dab of colour from the Brush tool on the Coloured Spots layer.
3. On the lineart layer, select the outside of the lineart, then invert the selection - this is to stop G'MIC filling the entire page.
4. Select the Coloured Spots layer.
5. Select "G'MIC -> G'MIC" from the Menu bar.
6. In G'MIC, select the filter "Black and white -> Colourize [comics]"
7. Set the filter options as per the example picture
- Input / Output: Input = Active and below, Output = In Place (default)
- Layers ordering: Input layers = Colour spots + lineart, Output layers = Extrapolated colours + lineart
- The Preview should show you how your colouring is working
8. Select "Apply" or "OK" to apply
9. Move the Lineart layer to be over the Colour layer.