Picture from here.
Not really sure what the Chosun Ilbo is basing this on, but it says those taxis for foreigners are "enjoying popularity":
Taxis exclusively designed for foreign tourists, which debuted in May, are enjoying popularity among their targeted customers.
The Seoul Metropolitan Government Monday said that the average daily number of reservations for these taxis from May 1 until June 4 was 15.3. Some 120 taxis are currently in operation exclusively for foreigners, and all of the drivers are fluent in English or Japanese. The average fare was W65,900 (US$1=W1,263), with 69.8 percent of all journeys being between Seoul and Incheon and Gimpo airports.
More in Korean here. I've done four other posts about them: here, here, here, and here. You can make reservations and get more information at the official website.