"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. ^.^