I would be concerned about fading of the color indicator on the strips. More ideal to take a picture if you really want to show the proof.
Agreed. I can't speak for all test strips, but I do know that allergen test strips will fade over time. pH strips I've worked with will also do the same. This would cause concern using this method as proof. 3 months down the road, I can imagine the confusion there would be when the indicators no longer present the same results because of fading.
I've never had anyone question our results. I always kept a log of test results (pass vs. fail), which allergen protein failed, and what piece of equipment it failed on. I would also keep record of re-tests. That always worked for my auditors, but as they say, pictures are worth a thousand words.