Yeah, I've been googling on how to do it too =)) It turned out to be very easy. For all of you who are trying to find out how to change the label colour of the system drive in the new OS X Lion, here is how you do it:
1. Secondary click on a drive -> Get Info
2. Find Sharing & Permissions and click a small lock icon
3. Enter your admin password
4. Then change the "everyone" user's permissions to Read & Write
5. Now you are able to change the label colour of the system drive. Change it to whatever you want. After that don't forget to change the permissions for user "everyone" back to Read.
Good luck =))