In the Inside and Outside tabs of the Layer Properties tool window, you can specify a solid color. The solid color can be used by coloring algorithms for special purposes.
To change the solid color, click on the Solid Color swatch in the Inside or Outside tab. By default, it is set to black, but you can choose any color. You can also change the opacity. By setting the opacity to 0, the solid color and thus the areas colored with the solid color will become transparent, so the lower layers will become visible.
By setting the Transfer function to None, the entire inside or outside area is filled with the solid color. This is useful if you do not want to use a coloring algorithm for that area. If you use a transparent solid color, the area will become transparent.
For example, you can use this if you want to color the inside area with a different gradient. Duplicate the layer and make the inside solid color in the top layer transparent. Now, you can change the gradient for the lower layer. Only the inside area of the lower layer, and the outside area of the top layer will be visible.
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