you only need to desaturise
images that have different tones in them. That image up there is green-scale, and can easily be colourised without desaturating first.
(
Damocles wants no part of your debauchery,
Wed 25 Oct 2006, 10:25,
archived)
clicking 'colourise' makes everything the same tone anyway
(
mugatu,
Wed 25 Oct 2006, 10:34,
archived)