[Orgmode] nicely managing multiple calendars in the agenda

[Eraldo Helal]

I have switched to using org files as 'calendars'. (events.org, university.org, 
etc)
I am using the agenda to view them.

However I am still having some troubles with 2 things:
1. different colors for different calendars (e.g. entries from events.org in 
orange)
2. toggling the visibility of individual calendars

=> 1
    Would it be possible to use different faces for different files?
    Or maybe even different faces for different categories?
    ,---- What I have:
    |[blue ] Saturday    1 May 2010
    |[black]   events:      Adam's party
    |[black]   events:      Magic show
    |[black]   university:  10:15-12:00 Management test
    `----
    ,---- What I want:
    |[blue  ] Saturday    1 May 2010
    |[orange]   events:      Adam's party
    |[orange]   events:      Adam's party
    |[purple]   university:  10:15-12:00 Management test
    `----

=> 2
    I do know that I can restrict the agenda to a single file,
    but how can I toggle the visibility of a single file? (e.g. exclude one 
file)
    Plus... Once I have toggled one file invisible...
    I would need some kind of overview of which files(calendars) are hidden and 
which ones are still active.

    One possible workaround would be using file-tags and filtering by them.
    While I do like the idea, it still does not solve the issue of knowing
    which files(calendars) are active and which ones are hidden at the moment.
    Plus I would be even happier if we could find a solution without using tags.

    A menu (just like the todo-kwd menu <C-c t>) would be ideal as an interface 
for toggling calendars
    and getting an overview of the currently displayed ones.