Per my research, it is not possible to format a calendar view with color hex codes.
JSON formatting capabilities are very limited for calendar view. You can only add additional classes to events using additionalEventClass and additionalEventClass only supports the naming conventions: ms-[location]-[color].
References:
https://learn.microsoft.com/en-us/sharepoint/dev/declarative-customization/view-calendar-formatting
https://devblogs.microsoft.com/microsoft365dev/mvp-article-using-theme-colors-in-list-formatting/
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