In order to make the Outlook calendar to show holidays in color, you can apply a category to it.

Procedures: (For Outlook 2021/2019)

• Open Microsoft Outlook and click to view your calendar.
• Select a holiday entry in your Calendar.
• Open the Color Categories dialog by tab Appointment/Meeting-> Categorize-> All Categories…
• Click on the category name: Holiday (not in Master Category List)
  


• Press New…
• Leave the name as it is but select a color for it.
  


• Press OK and OK again to return to your Calendar.
• Now all holidays will be shown in the color you choose.