The sample may turn purple during the test, or go to blue for a moment and then turn back to red/pink. This is called a "fading endpoint" and is caused by interference from metal ions. If this happens, do the test again, but this time add five drops of R-0012 before adding any R-0010 or R-0011L. Remember to count the initial five drops in the total.
In extreme cases, a fading endpoint may occur even when adding five drops of R-0012 at the start. If that happens, mix pool water with an equal quantity of distilled water, test that, and then multiply the result by two.