How to set up so on the Mac the default new user will have a single color wallpaper?
The only way I know how is to modify the System's "User Template". Here's the steps I used to test on Lion.


Use a current account to specify the desktop wallpaper you want. (If
you use "Solid Colors" there's a button in the lower right to pick a
custom color.
Copy the current account's com.apple.desktop.plist file to the user template:

sudo cp ~/Library/Preferences/com.apple.desktop.plist /System/Library/User\ Template/Non_localized/Library/Preferences/