Apple Color OneScanner 600/27 - Manual (Page 94)

Setting the image depth
You can change the number of colors used in an image or switch a color
image to black and white. You do this by changing the image depth.
To change depth:
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From the Image menu, choose an option from the Image Depth submenu:
If you change the depth of an image to fewer colors or grays, you cannot
recover the original image depth. You must rescan the original to get back the
higher number of colors.
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