Hi Euronate,
The PIXMA MX870 will use color ink tanks to print colored text. Even if the grayscale option is checked in the printer driver, if the original text is in a color other than black, the PIXMA MX870 will combine the cyan, magenta and yellow ink tanks together to make a composite black. This is why your color ink cartridges are still being used.
To use only the black ink tank, the original text must be black and the paper setting must be set to Plain Paper.
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