Can you change the color of the calendar drop down color other than the default theme color?
nope. It's an FM built-in that is not accessible in object styles.
alternatively, you can make your own calendar selection in a popover button (FM13) or window (<FM13). NinjaCal is a popular choice for the DIY calendar option.
On a Mac, you can edit the resourrces file - but you'd have to do this for every copy of FileMaker that would be accessing your database file, and it would change for every database file opened by that copy of FileMaker
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