Virtual Blueline: Floating outlines on a transparent layer

Now we'll make a new transparent layer to carry the black outlines.

Make sure the outlines are still selected.

Go to the Layers palette. As you did when making a new channel, go to the triangle at the top right corner of the palette, and pick New Layer... from the list that appears.

The New Layer dialog box appears. Make sure the settings are as shown here. Name the layer Outlines.

A new layer will appear in the Layers palette.

Pick the colour black from the Swatches palette.

Click the Paint Bucket on the outlines. The outlines on the new layer will fill with black. This may take a minute or so.
In the Select Menu, pick Deselect.
Save the file.

(Call up the Paint Bucket Options again and re-set the tolerance to 111; if it's left on 255, the Paint Bucket tool will just flood the whole image with colour every time you try to use it)

You can now colour on the base layer, without affecting the black outlines. If you have enough RAM, you can even add other layers, for shading, highlights, or special effects.