Image Processing Reference
4.4.5 Using Transparency
and the Colorize Filter to Color
Image Areas by Brightness
The Colors > Colorize function can also be used
to colorize image areas according to brightness
and to overlay them on the original black-and-
white photos using transparencies. This results in
an image with both gray levels and colored areas.
• Duplicate the Background layer of your
black-and-white photo in the Layers dialog.
This layer must be set to active.
• Access the Colors > Color to Alpha menu item
and select the From: [white] to Alpha option.
Click OK to accept your changes. The white
or bright image areas turn transparent.
• Access the Colors > Invert menu item to
invert the dark areas of the layer. In the
next editing step, the bright image areas
will appear much more intense than the
dark areas, while the transparent areas will
• Choose Colors > Colorize and proceed to
color the bright image areas of the layer with
any color you prefer.
• Finally, use the Dodge mode on the active
layer in the Layers dialog. This produces a
natural-looking overlay with the original
The coloring effect of the overlaying layer is opaque in Normal mode.
The layer mode of the overlaying layer creates an actual partial coloring
effect (after applying the Dodge mode, in this case).