From Ubuntu FAQ:
Will brown always be the default desktop colour?
Unlikely that ANYTHING will be static forever, given that we expect Ubuntu to be around a long time
Our current plan is that the Dapper Drake (Ubuntu 6.04 if we hit our April 2006 release date goal) will be the last of this first “set” of releases. So post-Dapper we have the opportunity to define a new “feel” or overarching theme. It would be unlikely to be… blue. But it might be substantially different to the current Human theme. For the moment, let's stay focused on the road to Dapper, polish up the existing Human theme to the max for that, and then break new ground post-Dapper.
From Mandriva FAQ:
*Why is the default desktop in Mandriva Linux blue?*
Because blue is the color of her eyes as well as the color of oceans and sky.
Blue is often the color of children's rooms because it cools them. Blue certainly has a connexion with human minds.
*Will blue always be the default desktop colour?*
While brown is currently a fashionable color on the desktop, blue is for sure part of Mandriva's color scheme since early days. Anyway, some of you have noticed that the Discovery version of Mandriva Linux provides an island picture as default background.
Last but not least, we provide a feature which lets users actually *change* the color of their desktop!