This needs a higher profile – a service to show you what any website looks like to someone with various sorts of colourblindness. (Example: compare WAI and colorblind WAI.) If you use the grayscale filter, not only will you know whether any bits of your page would be unreadable to one or more colourblind people, but it would also make it obvious where you haven’t explicitly specified colours (because the default colour will remain).
This is obviously useful to web designers, but perhaps it would also be useful to colourblind people too. When they complain about a website, they could send a URL saying “this is what it looks like to me”, to give the webmaster an idea of the problem.