Remember. It takes it's color attributes FROM YOUR CURRENT SMF THEME so check there (your styles.css) to make color changes!

I miss the full bodied feeling of ExtCalendar component but LOVE the integrated closeness of the forums (plus I hated ExtCalendars OBSCENE mixture of CSS and graphics for colors! ugh

)
what I would recommend doing is..
Look at the SMF theme demos, find their calendars and check 'em out - if they look cool enough, then download the theme, huntdown the calendar section of the theme and then cut and paste THAT into your theme.
(You may want to use the COPY THEME function of the admin menu first!)