Una soluzione parziale è suggerita su questo thread di Google :
There is a workaround that solved the problem for me.
The iPhone assigns calendar colours in
this order: red, orange, blue, green,
purple (then repeats that cycle). So
the first step is to switch off
Exchange syncing for Calendars in the
iPhone settings then go into the
m.google.com/sync web app and just
enable one of your calendars. Then
re-enable Exchange syncing. Now you'd
think it would colour that first one
red and it may do in your case but for
me it coloured the first calendar
orange, possibly because I had some
other local calendars already on the
iPhone.
If it colours your chosen calendar
with a colour you were not expecting,
just start again (disable
Exchange/change sync settings and
re-enable) and take whatever colour it
chose as the starting point in that
cycle above. Once you've got the first
calendar/colour combination working as
you like, go back into
m.google.com/sync and select the
calendar you want to go with the next
colour in the cycle. Wait a few
minutes. Check that it's showing up as
expected in the calendar app. And then
go back to m.google.com/sync and
repeat. If you take this one calendar
at a time approach, you can assign
colours as you want them, except of
course you will face problems if you
have say three green calendars and no
purple ones...