The goal of this tutorial, is to remove the white background from a piece of lineart.
So only the black lineart will remain.
Given the images transparency, as seen below.
I suggest that you use a very clean looking piece of line art for this tutorial.
Open your lineart in Gimp.
Select Layer>Transparency>Color to Alpha.
The Color to Alpha dialog will open.
The default color is white, which is what we want.
Click OK.
Your image should now look like the image below.
We need to create a background layer for our lineart.
Do this by Rignt-clicking on your lineart layer and selecting New Layer.
The Layer Fill Type by default is Transparency, that's what we want.
Click OK, to create the background layer. I named my new layer Background.
Your image will look like the image below.
We first need to move the background layer below the line art layer.
Do this by clicking on the Down Arrow in the Layer Dialog.
All we need to do now, is add a Color layer to our image and we're done.
Right-click the background layer, select New Layer. I named my new layer Color.
Click OK to create the new Color layer and start to color your line art.
Add more layers as needed, below the lineart, to complete the coloring.
Note:
All resources used in this tutorial are my own.