Very simply, this sets the transparency of one calendar's events to 40% (default).

Because calendars each use differently-named CSS classes, you have to do some work on this one, sorry. Edit the class name "ro-chip-20" to match whatever calendar you want to make transparent. (use the install parameters)

Find the name of the calendar with the selector tool.
In Firefox, hit F12 to active Firebug., select pointer pointing to rectangle tool,
then click on the calendar you want to change the color of.
The div you are looking for should look something like
You want the first value for class (ro-chip-20 or whatever it says for you).