Yes it is:
// event categories (can be modified by user)
$rcmail_config['categories'] = array(
'Personal' => '19F7FF',
'Work' => 'ff0000',
'Family' => '00ff00',
'Holidays' => 'ff6600',
);
// public calendar categories (can't be modified by user)
$rcmail_config['public_categories'] = array(
'Public' => 'ff6600',
);
// associated CalDAVs (can't be modified by user)
/* you can use here the same placeholders as in
'default_caldav_backend' */
$rcmail_config['public_caldavs'] = array(
'Public' => array(
'user' => '%u',
'pass' => '%p',
'url' =>
'
https://mycaldav.mydomain.tld/%u/events/public',
'auth' => 'basic',
'readonly' => false,
'extr' => false,
),
);
'Public' Caldav will be displayed in color #ff6600