Creating the Dark Patches on the Mouse

Now we need to outline the dark areas of the mouse, and fill them with colour. Create a layer called "darks" and position it above the gray rectangle and below the outline (using the LAYER option). Lock the "outline" layer.

Mark off the dark areas on the "darks" layer using the Freeform Pencil tool, do your best to close these areas - don't leave the lines open as we did for the ear outline. If your dark areas overlap the outline, thats not a problem. It's an illustration effect. The lines on the below picture are thinner because I increased the picture's size on the canvas.

Now, to colour the dark patches we need to select the actual colours from the mouse picture. Go to the "gray" layer and make it invisible by clicking the eye icon next to the lock icon. Select an area by clicking on the line delimiting it, go to the "Fill and Stroke" dialog, go to the "Stroke" tab and click on the button with the "X" (to make the outline go away). Go to the "Fill" tab and then select a colour using the colour picker pipete icon on the bottom of the vertical bar (or press F7). Repeat for all dark areas.

Next, clean up the shape of the dark patches by editing the path manually, or simplifying them with several presses of CTRL-L. Don't make these areas too smooth or circular - reflect the mouse's bone structure and shape.
Don't forget the eyes, like I did!

Skin Colour and Light Patches

Unless you'd like the mouse to be white, it needs a layer with skin colour. Create a layer called "skin", trace the entire mouse (or copy the outline we made originally) and fill it with a body colour you like. Make sure that the "skin" layer is above the "gray" and "picture" layer but below the other two. Use EDIT->PASTE IN PLACE if you decided to copy the outline to "skin" and fill it with colour.

Next, create a layer called "lights" and raise it to the top (but below "outline"). Make the "darks" and "skin" layers invisible using the eye icon so that you would be able to see the actual colours of the mouse. Remember that "gray" has been invisible the entire time - we don't need it anymore as the outline is finished.
Remember to highlight features such as muscles and bone, don't turn the mouse into a blob ;-)