There are a few ways to approach this.
One way would be to set the
Layer Mode of the layer you're coloring to
Multiply.
![Image](http://gimpchat.com/files/2101_Layer%20Mode%20Multiply.png)
Setting the layer mode to multiply,
lets the black lines show through.
However,
this may cause issues later on during the coloring process.
When other layers with color are added.
![Image](http://gimpchat.com/files/2101_separateurparagraphe%208zxyw.gif)
Another method would be to apply
Color to Alpha on the white of the image.
This will remove all the white from the image.
Leaving the black lines on a transparent layer.
The first thing I would do if useing this method.
Would be to make sure that the white is indeed as close to white as it can be.
This can be done by using
Color Curves.
In the example image below.
You can see that the white has some
grays/dark pixels.
![Image](http://gimpchat.com/files/2101_curves%20example.png)
By using
Color Curves,
this can be adjusted.
![Image](http://gimpchat.com/files/2101_curves%20example%203.png)
Then you can apply the
Color to Alpha of the white.
![Image](http://gimpchat.com/files/2101_color%20to%20alpha.png)
![Image](http://gimpchat.com/files/2101_color%20to%20alpha%203.png)