Question

why taxi nameplate is yellow

Answer 1

I think it is done to make it stand out from the rest of the vehicles.
It is not the same all around the world, so I don't think it is a universal thing.

Answer 2

Hey

Here is your answer,

It is represent the self driven taxis. It is not only used for private use but also money can be charged or paid in order to travel somewhere.

Hope it helps you!