Change balloons color

Hello,

I would like to be able to change balloons color at map/user. For example, I'd like to make them green by default and red for users not logged in the last (say) 7 days. Or even give the user the choice to be 'on vacation' and thus have their balloon set on a different color (like the status in a IM client).

Any hint on where I should start looking? I am not even sure this is a location or gmap issue.

Thanks for the attention.

Marco.