Opens a Change color dialog, where you may change the background data color for data associated with a particular window.
Example: The default data color of the Classes window, as determined by its tab color, is light blue. So when transactions are grouped by Class, those groups are displayed with a light blue background.
Note: In the Transactions window this menu item is named Change Groups color… and has a slightly different purpose. There, it lets you choose a background color for transaction groups which have no data color of their own.
Example: The Qty 1 column has no data window as do the Account and Class columns. Therefore Qty 1 has no tab color nor data color of its own. If the Groups color is set to light yellow, then when transactions are grouped by Qty 1, those groups will be shown in light yellow.