From Edie Freedman, Creative Director, O'Reilly, on 13 January 2011:
Web Design/Development: Teal
Perl: Turquoise blue
Java: Dark purple
Apple: Light purple
System and Network Admin: Dark blue
Windows: Royal blue
Data: Bright red
Mobile: Grassy green
Linux: Warm brown-red
In a Nutshell references: Rust red metallic
As topics morph and/or grow, we may introduce new colors and retire others. For example, you'll find some Data titles in the Programming group (we will change the color to our relatively new Data red when we reprint those titles).
-Edie Freedman, Creative Director at O'Reilly
Stormville's editorial comment: The perceptive reader will note these are Borgesian categories.