Outlook Calendar Color Categories Greyed Out. Just like in the solution you've mentioned, use contidional formatting for your calendar. By default, all outlook events are the same color.
I checked this out and that user claims that when the shared calendar appear in his outlook, it had the color categories showing (and claims that his categories list. Category colors you’ve set on calendar events in your microsoft 365 account in outlook desktop, outlook mac, or outlook web now show on outlook for ios in all your calendar views.