Hello,
The answer to this is that it depends.
For the most part color printers will use all color toner to produce grayscale output. But, some print drivers, like the Universal Print Driver, will allow users to modify settings so that ONLY the black toner is used for grayscale/black data.
One such setting is "Print All Text as Black". This will tell the printer to ONLY use the black cartridge to print all black/grayscale text.
Hope this helps,
JPac