On Tools menu, click Settings.
You can use the settings in your calendar views to improve the way various elements are displayed and, thus improve the overview or readability of your calendar. Color settings also apply to all the data record types that you can display in the calendar.
The Working hours entry refers to the times configured with the Working hours tab.
When you select a color with a drop-down list, the Windows default color window opens from which you can select any colour.