I have been diagnosed as colour-blind since the age of seven or so. I have taken the Isihara test so many times and always failed, especially on Red/Green colours. Until I was 59 and took the test in a different form.
Previously I have looked at cards in artificial light, but the last time I took the test six years back I was invited to look into a scope which sheltered my eyes from artificial light. To my amazement, I saw every number on the display for the first time. Talking to a colleague later I learned that he too saw all the numbers for the first time.
Is it possible that one sees colours more clearly when there is no background light? How is the test conducted during your examinations?