The green Honda belongs to one of the residents of the Buchanan house.
As for the cabbie's comment about the car color, we have never seen a full photo of Kim's car. The photo taken of Mangum just after putting her in it shows a cloudy looking patch on the side. I wonder if there is light gray primer on the front of Kim's car or possibly even white fenders or a hood from a wrecking yard.
Another possibility is language. Mostafa is a foreigner, and may not have his colors right. Here in CA, I have heard people misstate the Spanish name for the color black as "blanco", which means "white". Black is "negro", but I think "blanco" is mistaken for it because of its similarity in sound to "black". The mistake could be as simple as that.
One thing about the car color which I may have missed but I haven't read/heard--dark(er)/night lighting comes into play, and a car of a certain color can be mistaken for another shade altogether.