Three fails allowed in Australia.
Make sure you read the cards in good natural daylight
Last time for me it was a heavily overcast day and by fluorescent light
- four fails. Came back the next day. Read them all in broad sunlight.
Three fails. But only one was one of the previous day's fails.
Then the eye bloke said that the test can be fairly arbitrary.
He also said it is virtually unknown for colour perception to change with the passage of years.
Incidentally, he said that CVD people were used in the Second World War
in photo intelligence as they could study the recce photos
and that their CVD enabled them to discern camouflaged buildings
that non CVD people could not.
Maybe the PPRuNe expert in this field field, Dr Arthur Pape can comment.