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Depends mostly on the printer you are using, which will determine the settings available. In the Print dialog box, click on the pop-up menu that shows as Copies & Pages. Look for options that deal with color. For example, an inkjet printer I have shows a Color Management setting, with an option for "pure black and white" - that setting would print the light gray text as black. On the other hand, for an HP Color Laser printer, the Color Options doesn't provide any settings that are useful for what you want to do (even under the advanced options). The best I could find was in ColorSync (that choice will be present for all printers), choose "Lightness Decrease" from the Quartz Filter pop-up menu. That seems to darken the printed gray text a little bit.
Hope this helps...
