By default each Change type is associated with a color that is used in the Change Calendar.
This article has information on how to modify the default colors that have been specified to custom ones.
The Business Object ChangeTypeOfChange has a column called Frs_ColorCode that contains the value for the color to be used on the Change Calendar.
The Business Object ChangeTypeOfChange can be accessed from the Validation Business Objects in Configure Application/AdminUI.
Clicking on "edit" brings up a pop-up window with a Color Code field as a Color Picker type:
A new color can be specified to replace the existing color.