Which capital city in Central Asia restricts vehicles to only white cars on its roads?

Ashgabat is the capital city of Turkmenistan and is known for its unique urban policies, including the regulation that restricts most vehicles to only white cars. This regulation is part of an effort to promote a harmonious and aesthetically pleasing environment in the city. The choice of white is symbolic, often associated with purity and cleanliness in Turkmen culture.

Astana, now called Nur-Sultan, is the capital of Kazakhstan and has its own modern and distinct development but does not have such vehicle restrictions. Tashkent, the capital of Uzbekistan, is a major urban center with diverse vehicle regulations but does not impose restrictions on car colors. Bishkek, the capital of Kyrgyzstan, similarly does not have regulations pertaining to car colors. Thus, Ashgabat stands out as the city with this specific requirement, making it the correct answer.