Working with Black-and-White and Color Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 313

4.1.2 Removing Color Partly or Entirely

You can easily convert a color image into a black-and-white photo
by opening it in RGB mode and selecting the Grayscale menu
option. To toggle between RGB and Grayscale , just choose Image >
Mode in the image window. When an image is converted, the color
information is discarded and you're left with the bright-dark values
of the image in the form of gray levels. If you convert the image
back to RGB mode, the gray levels will remain, but you will be able
to colorize the image again.
When you open an image in RGB color mode, there are two
other methods to partly or entirely remove color from it. These
methods modify color saturation, making it unnecessary to change
the color mode of an image.
• Select the Colors > Hue-Saturation menu item (for individual
layers in the Layers dialog) and move the corresponding slider to the
left to reduce the color saturation of an image to pure gray levels, or as
desired.
• The Colors > Desaturate menu item discards the color information of an
image or a layer and reduces it to just bright-dark values (grayscale).

Figure 4.1
The Desaturate window

Have a closer look at the Desaturate function. It offers three options
to determine the grayscale value: Lightness , Luminosity , and Average .
The idea was perhaps to create a notion of variations in exposure and
developing time. The differences are not very obvious as you can see in
figures 4.2 through 4.4.

Figure 4.2
Grayscale using Lightness

Figure 4.3
Grayscale using Luminosity

Figure 4.4
Grayscale using Average