As the experts keep saying, accessibility isn't just about putting alternate text elements in your HTML so that blind people with screen readers can make sense of your site. One often-overlooked aspect is the use of colour, especially in the contrast of text on a background.
Giacomo Mazzocato has published a site with a tool that checks the contrast and brightness between colours and simulates some of the vision problems that users may encounter.
It takes a bit of getting used to, but it is a valuable aid. See it at http://gmazzocato.altervista.org/colorwheel/wheel.php.
Julian, many thanks for this very helpful link. The direct feedback about how colors are seen by different people is really impressive and this way very helpful when it comes to decide about colors for accessible pages.
Urs