Is it possible to set the background color for the first 15 minutes of an event, then to another color for the remainder of the time ( between start time and end time) in the BeforeEventRender event?
You could do that by adding a custom background image using e.BackgroundImage in BeforeEventRender.
If there are just a few possible combinations you can use a static image. It would also be possible to render a dynamic image using ASP.NET page.