如何在创建活动时设置Office 365日历颜色？

I am able to create events for office 365 calendar with php, but I am not able to figure out how to set the color of the event. My client wants different color for the event based on who is invited to the event. Be below code is the data that I currently use to set the event.

$event = array(
    "Subject" => $subject,
    "Location" => array("DisplayName" => $location),
    "Start"=> array(
            "DateTime" => $startTime,
            "TimeZone" => "Pacific Standard Time"
            ),
    "End"=> array(
            "DateTime" => $endTime,
            "TimeZone" => "Pacific Standard Time"
            ),

    "Body" => array("ContentType" => "HTML", "Content" => $htmlBody)
  );