Introducing background color for a custom login portal in version 11.0 of the KACE Systems Management Appliance.
Background color for Administrator, User and System login portals can now be modified in Settings | Control Panel | General Settings - Login Screen Options.

The color picker tool may vary from one browser to another.
Google Chrome

Firefox

Microsoft Edge

Please note, the color picker tool is not supported in Internet Explorer 11. For more information, please see here.