What i do is start with a sketch in the first layer. It will have a few circles and some basic lines.
Then i'll add another layer and in black, do the real lines i'll be using then i take away the sketch (don't delete it, just go to the little black circle next to that layer and click it; it'll go write and the sketch'll disappear) Then you'll have the lines with a checkured back ground.
Create a new layer and change it setting to 'multiply' (you can do this by clicking on the scroll bix at the top of the layers box) and in white - or whatever colour you want the back gorund to be - go over the whole drawing and it wouldn't rub out your lines!
Next, do the shading by again, creating a new layer and again, changing the layer's setting to multiply. change your colour to pale grey and just draw in the shading where you want it to go.
You should now have your lines with pale grey shading in some parts on a coloured/white background.
Now, create a new layer and change it's setting to 'subtract'. colour in your horse as you wish though it will go outside the lines (if your background is not white, you will need - in the background layer - to colour within the lines white

or colouring wouldn't work probaly (It'll come out a dark shade of the colour you want.)
Finally, go back to the shading layer and in
White (yes, white) colour near the shading and then blend the shading in to the white. The white will actually show up as the colour you've coloured it in the colour layer.
Hope this helps.