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I assume you are talking about the OTO chlorine test: a vial with a color chart, you add five drops, which normally turns the sample different shades of yellow, which are then matched against samples on the color chart. The OTO test is normally used to measure TC levels between 0 and 5. You can, however, use this test to get a vague idea of extremely high TC levels. If the sample turns a particularly vivid yellow the TC level is between 5 and 15, light orange is between 10 and 20, dark orange is between 15 and 30, and brown is 30 or higher.
Since your FC level is 18.5, the OTO test is expected to turn some medium to dark shade of orange.