Maybe they ran out of yellow paint or they are color blind. i have seen this before and wondered the same thing. i know this is not a true answer, but the temptation to reply overwhelmed me. sorry

Hi, I live in Melbourne, Australia and about ten or fifteen years ago, all taxi's where painted yellow. A couple of years ago some yellow taxis with green roofs started to appear. This was done to try to streamline the taxi business. The green roofed taxi could only operate during rush-hour. Here in Melbourne the "handover" from the day driver to the night driver is normally 5.00pm. This made it difficult for both drivers and passengers alike. The last thing the driver wanted was to drive 40 km's in the wrong direction (even if it was a good fare). Business people wanting to get back to the airport to fly back to their city found taxi's hard to find. Yellow and green taxi's come on at around 4.00pm and run into the night (I'm not sure what time). They both cover the handover gap and can be called upon during, say the Grand Prix or the Commonwealth Games, which will be held here in March. I don't know if that answers your question or not. The only other thing I can think of is that taxis are only allowed in certain zones. When I was in Washington D.C. I tried to wave down a taxi to take me back from Arlington cemetery to the Hilton Hotel, but he wouldn't do it because it was across the state line or something. He drove me to a taxi rank, paid that driver, jumped into another cab and finally got back to the hotel. Being a tourist you don't know the "system" and I'm sure we stand out like sore thumbs and are well and truely taken for a ride!