Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 238

You'll also have to apply this transformation to the egg layer in the Layers
dialog because the transformation produced a floating selection, which needs
to be anchored. Right-click the Floating Selection (Transformation) layer and
select the Anchor Layer item from the right-click menu, or simply click on the
Anchor the Floating Layer button at the bottom of the Layers dialog window.
3.9.4 Using the Paintbrush Tool to Create Lighting
and Shadow Effects—Painting in Glazing Technique
Lighting and shadow effects add depth, realism, and three-dimensional
effects to an image or object. There are several methods available to produce
such effects. One way is to simply paint them on.
Use the Fuzzy Select tool to create a selection across the red egg. The
selection is helpful as it prevents you from painting outside the contour of
the selection.
Figure 3.59
The Paintbrush tool options, the image, and layers for lighting and shadow effects in glazing (semitransparent) painting technique.