Ideally what you really need is a Text layer, and a color layer underneath that can be switched to dark gray for text layout contrast, or simply switched to transparent so that only the Text layer is visible, or printed. Unfortunately, Pages defaults to a white background, that you cannot dismiss in the application or the OS X print panel.
LibreOffice Writer defaults to a white background too. In its Preferences Appearances panel, one can change the document background color to 80% gray with a single click. Just right for setting a menu in white text. When you are done, and want to send it to your HP printer, the LibreOffice Writer printer panel has an option to uncheck document background. To test this, I chose PDF, and opened it in Preview, where from the View menu, I chose Show Document Background. I had white text on a transparent background — exactly what you want to send to your dark gray card stock.
LibreOffice is free. So is the LibreOffice Writer PDF documentation that was written with Writer itself.