It depends, and not on her eyes alone, or even mostly on her eyes, as you might suppose. The answer depends as much on records in the Bureau of Motor Vehicles. If 9 out of 10 taxis in the city are in fact yellow, and the rest orange, Granny's orange-taxi call will be wrong 9 times out of 13. Bayes' law tells us how to quantify the reliability of any new orange-taxi claim against background odds that heavily favor yellow. Correct observations of unlikely phenomena are themselves unlikely, even when coming from good eyes.
Incorrect eyes are horribly common.
Bayes' law, though, is counterintuitive, which is why it is routinely overlooked by amateur scientists, like the ones who write popular books or script TV news shows. A pair of perfectly sincere writers may see a bra behind every lump in a breast, just as Granny may report that all taxis are chartreuse. If they report few, if any, chartreuse cabs, or carcinogenic bras, the odds against Granny's view of things, or your local optician's, rise fast. So when the next pair of earnest unknowns publish a book advising you to burn bras, silicone, dioxin, PCBs or Alar, don't grab for a match too quickly. Bayes explains more than bras ever will.