On the Project Calendar each project is automatically given a randomly selected color to uniquely identify the project on the calendar. Sometimes, the calendar will select a really ugly color that makes the text hard to read. If you want to or need to change the project color to something that is easier on the eyes you can change the project color by clicking the colored task counter next to the project name.
Once you click the task counter you will be shown a color picker to pick and choose the color of your choice.