Amber indicators were introduced 1 October 1959. So all FEs and most FCs had the white ones. Maybe you've got one of each because they were doing some overtime in the factory when they built your car, midnight Sept 30
It's more likely whoever had it before you wasn't able to source one the correct colour, and got whatever they could get.
If the brake lights flash, the lower lens
should be white.
If the lower lens flashes, it
must be amber.
It's not legal to have flashing white rear indicators. Some people who want their car to look right but be a bit safer (ie have the woman driver
behind actually realise you are turning right) use white lenses and fit amber globes.
Hope that helps.
More info here:
http://www.fefcholden.org.au/techinfo/fcpaintstyles.htmlcheers
RET