The apparent reddening comes from light being more readily transmitted through the liquefied substance. The emotional appeal of a good story is no indication whatsoever that it is true. Given the number of claimed miracles, this abysmal track record casts extreme doubt on the veracity of any purported miracle which may still remain unexplained. I fully realize that not every story of a miraculous happening is worthy of belief, but these are incredibly well documented and have never, so far, been proven to be caused by anything found in nature. In addition, claims are not documentation of the claimed events.
Proper documentation includes solid evidence and no claimed miracle has ever provided any.
Now, our knowledge of human anatomy and physiology is not perfect today and was certainly shaky in centuries past. It must be acknowledged that a report of some miraculous healing or other event many centuries ago is likely to be a result more of human ignorance than of the supernatural. I should hit one more point about Fatima. In the original reports, the rainstorm happened during the previous night while the faithful were gathering. The attentiveness of the crowd watching the Sun and eagerly anticipating the predicted appearance of the Virgin Mary would certainly make it seem as though time had passed quickly.