I'm afraid the calendar is not very customizable. There is no option to choose different colors in the week view. The only place I see different colors is in the Agenda view, and those colors are based on the colors of the email address associated with those entries.
Lack of customizability in the calendar has been a sore point with me for years. As an alternative you may want to take a look at Pocket Informant by WebIS
. I've tried it and while it's the only real alternative our there, I'm not a big fan.