GrayBit is an online accessibility testing tool designed to visually convert a full-color web page into a grayscale rendition for the purpose of visually testing the page’s perceived contrast.
Regardless of a few very minor bugs, the converter works very nicely, dealing with images referenced in the HTML, as well as any images and colours defined in the CSS. Moreover, all link references in a page are changed to run through the converter as well, so that you can do an entire run through a site and view the converted version of each page…slick.
It’s worth noting, though, that, as useful as this tool is, it still only provides a way to carry out a subjective assessment of a page’s contrast. A human tester running pages through the converter still has to make a personal judgement call on whether or not contrast is sufficient. Also, this tool does not (yet?) simulate the effect of different types of colour blindness (which not only influence the hue, but also the perceived brightness of colours). And lastly, the mathematical conversion to greyscale used by GrayBit may not complete match the subjective brightness perception of the original colours; for instance, compare the top banner on Jona’s page in its original and converted forms - the greyscale version of the gradient appears darker (to me, anyway) than the fluorescent green one, giving the impression of a far better contrast than originally present.
All in all, though, this is an excellent tool to demonstrate the reason why a site shouldn’t
rely on colour alone to somebody who may not even be aware of the potential problem encountered by users with colour blindness. Use it in conjunction with Gez Lemon’s colour contrast analyser for slightly more objective results.