[29 June 2009]
The XSD Datatypes spec has a diagram showing the hierarchical derivation relations among the built-in datatypes. The old version (created by Asir Vedamuthu, to whom thanks, and used in XSD 1.0 and in earlier drafts of XSD 1.1) 
has simple color-coding to distinguish various classes of datatypes (what are now called the special datatypes, the primitives, and the other built-ins).
For the Candidate Recommendation draft of XSD 1.1, though, we needed to make a new drawing to show the built-in datatypes added in 1.1 (anyAtomicType, dateTimeStamp, dayTimeDuration, yearMonthDuration, precisionDecimal).
The new version created for the Candidate Recommendation draft has a new color scheme, which I made with the help of a very nice tool for color, now to be found at colorschemedesigner.com (I used the previous version, but the functionality I counted on is still there). This tool (and some others) allows you to see an approximation of the effect of your color scheme for a reader with various forms of color perception deficit (protanopy, deuteranopy, tritanopy, etc.), 
which means you can try to ensure that the distinctions in your diagrams are visible also to readers with those forms of vision.
I found it remarkable that I ended up with a color scheme I find more attractive than the old one; it’s remarkable how many people have told me they think the same (without realizing the proximate cause of the change).
SVG, of course, makes it easy to make diagrams for which the color scheme can easily be modified. And XSLT makes it easier to generate this diagram and to modify it systematically in various ways (including color scheme). But it’s the idea of universal design that gets the credit for making the diagram visually more attractive.
Universal design: try it sometime. You’ll be glad you did.
IMHO the most important form of color-blindness is the secondary achromatopsia exhibited by those of us who like to print our standards documents on black-and-white printers that insist on gray-scaling everything. It’s maddening when stand-out red or green or yellow appears as gray, light gray, and faint gray respectively. Always check how things look in black and white.