ow_system_plugins/admin/components/authorization_role_edit.php :
$colors = array(
'#999999', '#85db18', '#a7c520', '#046390', '#db4105', '#ff9800', '#01a2a6',
'#29d9c2', '#dc3522', '#1a9481', '#003d5c', '#046380', '#f23005', '#8b0f03',
'#2f6d7a', '#70a99a', '#b6d051', '#b52841', '#ff8939', '#e85f4d', '#590051',
'#303133', '#585956', '#99917c', '#ccc794', '#e66a00'
Just replace one of the color codes by yours and save changes.
Once this is done, your color should appear instead of default one, when you will be selecting label color for certain user role in admin panel.