property Calendar.NonMonthDaysColor as Color
Retrieves or sets a value that indicates the color to show the non-month days.

TypeDescription
Color A Color expression that specifies the color to show the days that are not owned by the current month. 
By default, the NonMonthDaysColor property is gray. The NonMonthDaysColor property specifies the foreground color to show the days that are not owned by displaying month. The ShowNonMonthDays property specifies whether the calendar displays the days that are not-owned by the displaying month.  Use the FirstVisibleDate property to get the first visible date. Use the LastVisibleDate property to get the last visible date. The NonworkingDaysForeColor property specifies the foreground color for non-working days.

The following screen shot shows the non-month days in red: