Generate different colors for users (color dates in calendar)

Generate different colors for users (color dates in calendar)

I have a only one calendar and many users who create events in that
calendar. Each user shall have an unique color as background for all the
events he creates.
Users are unlimited, but in any case less than ffffff(16777215) users. So
i just need an algorithm that creates an unique color for each user that
is added to the calendar. The less users the calendar has the more
diffrent should the colors be; e.g. red, green, blue, yellow for 4 users,
but for 50 users of course the colors become more close to each other.