“Originally, hackney carriage companies were distinguished from each other by their drivers’ livery (uniforms) and by the colours of their carriages. For example, at the end of the 19th century in Paris, Compagnie Generale carriages were painted blue, while those of Abeille were painted green (“The Paris Cabman”). During the early years of the twentieth century, private cars were usually black because paints of other colours were not durable. Taxis were the exception, as they would be touched up or worn out. Around the world today, taxi companies are still distinguished by the way their cars are painted.” (from http://en.wikipedia.org/wiki/Taxicab)

There’s a common belief that a lot of taxicabs (especially when they first came about) are yellow because yellow is quite an uncommon color among cars and stands out more. Even today, the color schemes of taxis are such that they stand out (green, red – mainly bright colors).

Even taxi companies see the value of standing out and being noticed.