Select an object, then select the “Fill” tool from the toolbar.
You can add basic colors from the menu or create more custom colors using the color gradient. You can also use a hex code or swatch tool to more accurately match colors of other elements within your diagram.
You can also choose the kind of fill pattern at the top of the “Fill” menu.
Editor information before 6/27/18: https://support.cacoo.com/hc/en-us/articles/212778448-The-Editor