One of the shortcomings of the built in SharePoint calendar list is that it does not support colour coding of event items. This is an often requested feature by end users, they are used to the way Outlook on their desktop works and naturally expect the SharePoint calendar to work in the same way.
Help is at hand with the PlanetWilson SharePoint Colour Calendar!
We are happy to offer this solution free of charge to the SharePoint community, instructions on installation can be found here.
The SharePoint 2013 version of the calendar can be found here