Paper towels are not designed to remove the micro-particles that alter your ph, and will not itself alter the ph. (unless the paper towels have been bleached by the manufacturer) Better than nothing...but not by much.
And as you know...any tinting skews color-coded results.

What was your before and after results, and was the liquid honestly clear? If so...what kind of paper towels are you using? (they might be bleaching the water, lol.)

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