Folder colors are not supported in classic Outlook, either directly in the UI or via the built-in color category system.
Classic Outlook supports color categories on items (mail, calendar, tasks, etc.), but not on folders themselves. The feature to color folders exists only in the New Outlook, as indicated by the feature comparison where “Folder colors” are listed as “Not supported” for Classic Outlook and “Available” for New Outlook.
VBA and the Outlook object model expose color categories and item-level formatting, but there is no supported property or API to assign a color to a folder in classic Outlook. Any folder-level coloring seen in New Outlook is not available through classic Outlook’s extensibility model.
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