I tried to use the color labels of appointments and/or calendars for the print preview.

But when trying to access the colorLabel property in the template files (e.g. /opt/open-xchange/templates/cp_monthview_table_appsuite.tmpl) it shows me an error:

TMPL-0002 Categories=ERROR Message='The underlying templating system threw an exception: The following has evaluated to null or missing:
==> appointment.colorLabel  [in template "cp_monthview_table_appsuite.tmpl" at line 172, column 48]

Tip: If the failing expression is known to be legally null/missing, either specify a default value with myOptionalVar!myDefault, or use <#if myOptionalVar??>when-present<#else>when-missing</#if>. (These only cover the last step of the expression; to cover the whole expression, use parenthessis: (myOptionVar.foo)!myDefault, (myOptionVar.foo)??

The failing instruction:
==> ${appointment.colorLabel}  [in template "cp_monthview_table_appsuite.tmpl" at line 172, column 46]' exceptionID=106665063-2936
How can I make the color-Label accessible in the template files?