Most inkjet printers use all colours even to print in black and white. Sounds crazy, but actually you get better halftones when a mix of CMYK (cyan, magenta, yellow, black) is put down instead of just black.
If you view a print of a greyscale image under a magnifying glass you can see the rosette pattern of CMYK making up the dots. Fonts are anti-aliased (removal of jaggies on edges) and to do this requires halftoning which also will use all the colours to get the best halftone effect.
So there you have it. That's why ink jet printers need colour cartridges to print in B & W.
In fact even brochure images printed in just greyscale use all CMYK colours if that is specified for output. You can try this in Photoshop. Open an image, make it a greyscale, output CMYK separations; look at the C, M, Y layers and you will see they aren't blank.
Last edited by: car4play on Wed 21 Apr 10 at 19:03
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