You can customize the colors of events by setting the Colors Field property of the Schedule component in the Lightning app builder.

Enter the API name of a field on Reservation Template, which outputs three hex or css colors separated by commas.

So for example, the following are valid values: "#000000,#121212,#ffffff" or "white,red,black".

The first value represents the text color, the second value the background color, and the third value the border color.