Well, my guess is that you where colouring on the wrong layer. The layer with your lineart on was not transperant, so the white background would hide the colour.
The drawing works fine to colour though,
here's what I did^^.
1. I set this layer (the lineart layer) to Multiply (as I showed in my last post).
2. Then I added a new layer below the lineart layer, and coloured on that. As you can see the colour shows up but it doesn't hide the lineart.
3. The + sign button where you add a new (transperant) layer.
Next time when you make a layer, start by adding a new layer by clicking the + sign button, then you draw your lineart on it. To make the colour show up you have to colour on a layer below the lineart. You can move your layers by draging them (click and hold)with your mouse.