Canon imageRUNNER C3100 N Printer Manual page 90

The gray data appears grainy.
Cause
Remedy 1 Disable RGB Pure Black Process and Prioritize Printing of Line and Text in the
Remedy 2 Set RGB Pure Black Process to 'Off' and Output Profile to 'PS Normal' on the
NOTE
4
If you change the above settings, gray or black data is printed using CMYK toners.
Therefore, the data may be printed in color, even if all pages are monochrome.
4-14 Printing Problems
The RGB Pure Black Process setting, in combination with the Prioritize Printing
of Line and Text setting in the printer driver or Output Profile setting on the
control panel, is affecting the gray part of the data.
Windows PS printer driver or Mac PS printer driver and print the data again.
(See the Help function on the printer driver.)
PS menu from the touch panel display and print the data again. (See "RGB
Pure Black Process," on p. 2-37, and "Output Profile," on p. 2-35.)