Nitrate Reduction

[TheSuN]

Hello, I'm new to this forum, plus I have a question to ask: Nitrate test
How long does the red color have to stay after adding reagents A and B to be called a positive test.

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Here is what I did in lab:

After incubation, I did the Nitrate test by adding the 2 reagents indicated in my lab manual.
The tube turned red (which should mean (+) for nitrate reduction)
However, the color quickly reverted back to the normal broth color before the addition of reagents A and B.
Every time I tried adding more reagents, the same thing happened: the red color never remained for long;it did turn red, however.

I gave up on the reagents and tried adding some powdered Zinc.
The result was colorless (this should also mean (+) for nitrate reduction)

So, based on my description, would you please tell me if this is a positive test for Nitrate reduction?