flags
Red, yellow, blue or every other colour from the colour spectrum. The green flag forms the checkered or striped goal on the green. This is also the most important function of the flag, signing where the hole is.
Colour of the flag
Who ever wondered why the colour of the green flags differ. This has something to do with old golf courses. In previous times, it was not yet decided to construct two circles of 9 holes around the clubhouse. But holes where laid in a long line. Golfers played two times on the 8 centre holes. 9 holes out and 9 holes towards the clubhouse. To avoid confusion on the greens that had to be played double between ‘outgoing’ and ‘ingoing’ golfers, the holes where marked with a different colour.
Nowadays the colour of the flag is more an esthetical choice.