This reminds me of the day I was told to select all taxis... took me some time to understand they meant "yellow cars" (taxis are white here)
Taxis are a different color everywhere, there's no "standard," even NYC has green taxis ("Boro Taxis"). Yellow is a popular color, because it stands out, but I've taken taxis every color of the rainbow (well, maybe not pink or purple), each area or company is different.
In my neighborhood two companies operate taxis, one has a fleet of red taxis and another has a fleet of blue taxis. From my observation, black is probably the second most common taxi color.
You identify a taxi by it's markings, not it's color.
I mean you tell me to check the markings, but you know in those captchas you can't see any markings because most things look like a blurry blob.
New York City also has green taxis, so it's not anywhere-centric thing.