The colour of the RosterSMS calendar on Apple devices is selectable via a setting on the RosterSMS website under the Settings for Calendar (direct link: https://rostersms.com/settings-calendar/ )
Most Android devices will follow the colour setting but results vary depending on the particular Calendar app. Desktop/laptop calendar clients may be selectable from within the calendar software itself.
After saving your colour change on the Settings page, wait at least 1 minute before attempting to refresh the calendar. It can take up to 60 seconds for the calendar server to sync to the Web site colour setting.
If you are unfamiliar with the steps to refresh the calendar on iPad visit the FAQ how-do-i-refresh-the-ipad-calendar
Note: Since the calendar is read-only to avoid accidental changes, the colour of the calendar is also read-only. Therefore the colour has to be changed via the website.