How do I change the color of the indicator applet to green?

The indicator colouring is a deliberate change by the Canonical design team - see here

To summarise -

• Red: denotes critical issues
• Orange: denotes critical warnings
• Blue: denotes pure information.
• Green denotes that something is "now OK", or "approved".

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.