(The color in the screenshot is for testing purposes only, you can set NotifyOSD to any colors you like)
As you probably know, the NotifyOSD notification bubbles in Ubuntu don't support any kind of customization. Mark Shuttleworth himself said that it's ok to customize the NotifyOSD appearance
(so we might get that in Ubuntu at some point), but not the behavior.
But that doesn't mean you can't customize both the NotifyOSD notification bubbles appearance and behavior if you want. You can do it with the patched NotifyOSD from Leolik's PPA which has just been updated with Ubuntu 10.10 Maverick Meerkat packages (but it also works in Ubuntu 9.10 Karmic Koala and 10.04 Lucid Lynx - like we've previously wrote).
The patched NotifyOSD allows a lot of customizations such as changing the position on the screen (corner to display the notification bubbles), colors, timeout and much more.