Two points laminating might alter the colours, not sure but you would need to be careful
Second point and most important THE COLOURS MUST BE OBSERVED IN DAYLIGHT, failure to do this will give a false reading, Subdued but good daylight is the best, not strong sunlight the colours are too difficult to see.
All artificial light has differing colour temperatures and will alter the colour on your work—-don't let anyone try to convince you otherwise I have been tempering small steel parts for many years and to my cost have found this to be very true.
Closure chart is very good though.
John