Nikon D810 - Manual (Page 416)

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Copy photographs in Black-and-white,
Sepia, or Cyanotype (blue and white
monochrome).
Selecting Sepia or Cyanotype displays
a preview of the selected image; press 1
to increase color saturation, 3 to
decrease. Press J to create a
monochrome copy.
Monochrome
G
button ➜ N retouch menu
Increase saturation
Decrease saturation