Try Custom Color option available.
Change a calendar’s color
- In the Calendar app
on your Mac, Control-click the calendar’s name in the calendar list. - If you don’t see the calendar list on the left, choose View > Show Calendar List.
- Click a colored circle to select a preset color, or click Custom Color to select a color from the Colors window.
Note: You can’t change the color of some calendars, such as the Birthdays calendar and the Siri Found in Apps calendar.
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Drag the crosshair in a circle to select the color.
Change a calendar’s name or color on Mac - Apple Support