Colorless for Firefox by hereafter is perfect to preview a page for later B&W print. I'm using it with Invert Image Color by vycb, that inverts selected images like for example a black console window image, becoming white background with black fonts, saving a lot of black ink when printing a webpage. And finally Print as PDF with Firefox.

A pitch for Hereafter:
You know when users have a color printer, one of the colors is less used and the others are almost over... Wouldn't it be great to print a webpage using mainly that less used color?
Your Sepia option idea could be expanded to Cyan, Magenta and Yellow! Well... maybe not yellow yellow, maybe your Sepia is perfect for this last one. You decide.
This would give the users a way to spend their less used color in the printer. Thank you.