Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 203

Figure 3.27
The Landscape layer with deleted sky (transparent area)
Take a closer look at the image: Since the border of the selection was
feathered, the contour of the image that remains is feathered toward
transparency. The transition is smooth and natural looking.
In the following exercises, you will use GIMP tools to select colors and fill
layers or selections with a color. These will be described in more detail in the
following sections.
Briefly, you will do the following:
• Use the Color Picker from the Toolbox to select a light-blue shade as the
foreground color.
• Make the Sky layer your active layer.
• Select the Bucket Fill tool from the Toolbox and click on the image. The Sky
layer will be filled entirely with the selected foreground color.
• If your entire image turned blue, it happened because the Sky layer is on
top of the stack. Move the layer underneath the landscape by clicking the
Lower Layer button in the Layers dialog.
• You can use the Dodge/Burn tool to darken the mountains in the
background. Set the Burn control in the tool's options. Set Opacity