The Custom Color button lets you apply a custom color to the current selection in conjunction with the current Color Mode.
The Custom Color button
To change the color of some selected objects to a custom color not in the color palette, first make sure the proper Color Mode button is depressed, then press the Custom Color button. The standard Windows Color dialog box will appear. Choose a color and press OK to complete the process.
The Color property can also be accessed through the various property pages. For more information, see Properties.