I want to change the indicator app's color from "blue" to "green" again. In 10.04 all notifications were in green, 11.04 now has blue color notification.
How can I change it back to green?
The indicator colouring is a deliberate change by the Canonical design team - see here
To summarise -
Hence the message indicator was changed to blue to indicate just "information".
Thus, I think the only way to change back to green is to replace the ubuntu-mono package with a version from maverick or lucid. Not sure whether this is advisable though.
Instructions are available under this link. This works for both 11.10 and 12.04.
Also, you may find this post on OMG!Ubuntu! useful.