"The taxi is a yellow car."
Translation:Το ταξί είναι ένα κίτρινο αμάξι.
The decision about the taxi colors is taken locally. Take a look at the link for every city http://taxitzhs.blogspot.gr/2012/01/blog-post_7060.html?m=1
I don't think the phrase is aiming for Athens here, it's just that yellow is a pretty popular color for taxis.
In other cities of Greece, like Βέροια, Αλεξανδρούπολη, Άργος, Καστοριά, Σέρρες, Κιλκίς, Πρέβεζα, and non Greek cities like New York, taxis are also yellow. ^.^
But here in Greece's most beautiful city, Thessaloniki, they are blue and white
That would be even more specific.. We can't please everyone, can we? xD
"το ταξί είναι κίτρινο αμάξι" is also currently marked as incorrect - if you'd be so kind as to do the honours.