In OS X it’s possible to change the highlight color. You can do this in the General ▸ Appearance panel in System Preferences. The Calendar application doesn’t use the highlight color for ‘Today’. This uses the default color blue, you can enable the system highlight color for the today selection with the following command.
defaults write com.apple.ical CalUIUseSystemHighlightColorForToday -bool TRUE
Revert to default setting with:
defaults delete com.apple.ical CalUIUseSystemHighlightColorForToday
Not working under 10.9.1, even after system restart.
You are correct, it doesn’t work under Mavericks Calendar – pity, I used to like this colour selection