According to Chrono-tm.org an Austrian based website ran by those who oppose the regime of President Gurbanguly Berdimuhamedow of Turkmenistan, Customs officials have refused to allow the importation of black vehicles. President Berdimuhamedow has started using his own fleet of white limousines, apparently the switch to white cars was also done by 160 of the country's government officials and the heads of media outlets according to Russia's RIA Novosti new agency. White is considered a lucky color and is just another one of the odd vehicular rules in Turkmenistan. The country also bans the importation of coupes, supercars (with the exception of the Presidents bright Green Bugatti) and does not allow personalised number plates or tinted windows.
It seems that the President has decided that white is the color since he has been spending billions of dollars remodelling the capital into "the white city" by covering major buildings with white marble.