Hi @Austin
Welcome to our forum!
Please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client. According to your description, it seems to be about Graph. We're not very familiar with the Graph API. The following is for informational purposes only.
As far as I know, as a delegate to view the mailbox owner's color categories, the mailbox owner needs to assign you at least “Can view all details” permissions (Microsoft 365 Exchange Online) or “Reviewer” permissions (Exchange on-premises).
Reference Articles: https://www.msoutlook.info/question/color-categories-in-shared-and-delegate-calendars.
(Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
Hope this helps!
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