Sorry to dig up this really old question, but I've got pretty much the same question.
It's possible to change it to a different colour in Leopard. Sort of...
You can make the blue screen last for half as long by editing the value for
Root > Colors > FrenchBlue
in this file:
/System/Library/Frameworks/ApplicationServices.framework/Versions/A/Frameworks/CoreGraphics.framework/Versions/A/Resources/Configuration.plist
(see here)
Changing that will mean when you turn on your mac, you will get a grey apple boot screen, a blue screen, and then your custom-coloured screen.
At the moment I'm trying to figure out how to customize the colour of the first blue screen as well. I think it is related to the blue screen that fades in and out when you change resolutions or plug in an external monitor. So if anyone knows where that plist is, please let me know...
Natobasso, that's not what I was talking about but it does make the boot process a bit more unified, especially if you use the login screen.
What I'd really like to have is a light grey screen from power on right up until my user wallpaper loads. I don't like how it has a grey screen then a blue screen then my desktop wallpaper.
Somewhere in the system files there is probably another 'French Blue' (or maybe under another name) defined in a plist that I want to change to the same grey as the apple logo boot screen. There are two. I've found one of them (as above) but the other seems to be somewhere else.
I think I've got something against aqua at the moment...