So far there has only been one question asked frequently enough to warrant a write-up. The following text is from the most recent email I'd sent on the subject, and was copied to this html document when the next person asked the same question.
What's with the 'coverage' filters?
Why do the logo terms of use say my page should be either
all one color or entirely unchanged under those filters?
The idea of the 'coverage' tests is that they check what parts of the site the colorfilter can manipulate. Generally, any color that you explicitly specify can be manipulated by the colorfilter.
If you explicitly specify all colors, then the coverage test will demonstrate that it was able to reach all colors by first turning every color white, and then turning every color black. You won't be able to see any content because all you'll see is a white page and then a black page.
On the other hand, if you do not specify any colors explicitly, then the colorfilter won't be able to filter any of them. When you use the white filter, the page will look as it normally looks. When you use the black filter, the page will look the same.
Either of these cases is fine.
The problem cases are when you explicitly specify some colors, but not others. For example, if your site's stylesheet specifies a default color for text (dark gray, for example), but fails to specify a default background color, then you may run into trouble. If the user's web browser defaults to a white background, then you should be fine. However, if the user's browser defaults to a gray background (as many Netscape browsers did, and may or may not do anymore), then you will be showing the user dark gray text on a gray background. This is a problem.
The colorfilter coverage tests would reveal this problem. When you used the white coverage test, you might see the site as white text on a white background. The white background would not be because the colorfilter turned the background white (because you never specified a background color, in this example), but rather the white would just be your web browser's default background color. When you used the black color filter you would see black text (because the colorfilter turned your 'dark gray' into black) on a white background (because that is your browser's default, and you never specified a color so the colorfilter couldn't change it). The fact that the background was white in both of the coverage filters means that the colorfilter couldn't change that color, and the color is determined by the browser.
When you set the text color but fail to set the background color, then you're just hoping that your user's default background color won't conflict with your design. Mozilla lets advanced users use their own custom default stylesheets, so there's no way that you can be certain what someone's default colors might be. It is always best to either let the end user specify all foreground/background color combinations, or to specify them all yourself. It isn't good to specify just the foreground (or just the background) and cross your fingers and hope that things will work out on the user's end.
So, the convoluted second condition is a way of checking for this sort of problem.
If you let the user make all color decisions, then your code won't specify any foreground or background colors. The colorfilter won't be able to filter what you didn't specify, so the content will be legible using the white coverage test and the black coverage test. This is fine.
If you specify all foreground colors and background colors then the coverage tests will show you a plain white page and then a plain black page because the filter was able to reach all of your colors. This is fine.
If you specify only some of the colors, then the page may have content showing up under either the white or black coverage test, but not under the other of the two. This is not OK. It means that you're gambling that the user's preferences will not conflict with your color choices in a way that renders the content unreadable.