When setting the Account Color options under Preferences > Accounts, the option to 'Show the Account Color on tabs' updates in real time as you toggle the setting. The same is not true for the option to 'Show the Account Color as background' as it only updates upon exiting the preferences dialog. As that dialog does not have an 'Apply' button and only a 'Close' action button, the fact that this setting does not update in real time could be confusing for the user, and makes the option appear at first to be not-effective at all.