How long are you letting the ammonia test settle before you attempt to read it? I found mine always has a greenish tint to it until 5-10 minutes later. Then it becomes easier to tell that it's yellow and not green.
Seeing your results for so long now, i wouldn't be worried to go longer without any water change. Just keep on testing and see if ammonia really goes up.
Also, you can try keeping an ammonia test tube full for a day and test again the next day to see if they are any different. I believe it will hold its color long enough. Also, I don't remember if you tested your tap water for ammonia? Would also be a good sample to compare with.
Another option would be steal a little bit of water from a healthy looking tank at your LFS and test it. There shouldn't be any ammonia in that one!