One of the core principles of access for people with disabilities is that information should be understood even if the user can't see color.
Mac-using Web developers can test their pages for color dependence by viewing pages in grayscale. Here's how you do that in Mac OS X:
This technique can also be used by application developers to ensure their apps meet Apple's Human Interface guidelines for color dependence. The Universal Access settings are actually quite useful beyond simply enabling access by people with disabilities.
Mac OS X Hints
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