Delineate your project. Figure out what sorts of elements comprise your creation, and split up your project into those elements. For example, if you wish to create an expense chart, you can use separate categories for labor, rent, advertising and a slew of other categories.
Match a color to each type of object. For example, all labor costs could be yellow.
Create the key. You should write the type of item in the corresponding color. Alternatively, you can write the type of item and then add an equal sign, writing out the color you will use for that object.