Hello @Q Dao,
Welcome to Atlassian Community! It's nice to have you.
I believe you are referring to the current yellow highlight color on today's date, and not the colors for each event type. Is that correct?
If so, it is not possible to customize that color. It would require you to modify the source code, which is not publically available for Confluence Team Calendars:
There was a request in the past to customize the colors using CSS, but it has been closed as we do not have plans to implement this in the near future:
Thank you for your understanding. Please let us know if you have any further questions about this.
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