While the Green Kangaroo looks pretty much the same in both Mode/Greyscale and Adjustment/Desaturate, note how much of a difference there is in the bird on the left.
Alternation: increase Red, Green, Blue; decrease Yellow, Cyan, Magenta
Increase moves the luminance of a particular color towards white, decrease turns the color black.
The contrast on the Kangaroo is slightly higher, but notice the darkness of the bird's beak!
Alternation: increase Yellow, Cyan, Magenta; decrease Red, Green, Blue
The Kangaroo is practically monochrome; little difference between dark body and light belly
Alternations: increase Cyan, Blue, Magenta; decrease Red, Yellow, Green
The Kangaroo belly has actually gone darker than the dark body
Alternation: increase Red, Yellow, Green; decrease Cyan, Blue, Magenta
The Kangaroo belly has gone nearly white, while the body is almost black; very high contrast
