Color Gray scale
Converts the image from color or indexed-color to gray scale (luminosity). The relative contributions to luminosity from the red, green, and blue components are determined by the ``Luminosity factors'' which can be changed by selecting ``Color...Color settings''.
The default is:
The results for 8-bit images may be different from the results of merely changing the colormap to gray scale, because for 8-bit images, the conversion creates a new colormap to match the luminosities in the image as closely as possible.
Gray scale color
Deactivates grayscale mapping, reactivates color manipulation features, and tries to restore previous color information if possible.
WARNING: Converting color to grayscale and back is not always a reversible process.