Saved the image too big the first time.

The orange and black ones belong to Yellow Cab Company, which is one of the largest companies. Yes, I know they're not yellow. I don't know why.

Here's one of the most surprising taxis I've seen; I'm terrible with Volvos so I'm not going to hazard a guess as to age (picture taken April 2011):

Nice. Sad to see these replaced by Korean cars indeed

This sort of reminds me when Syrian border taxis that were used to travel to Lebanon or Jordan, consisted of a fleet 1970s Mopar cars. They were used until the late 2000s (circa 2008/2009) when they were replaced by crappy Hyundais and Kias. Not sure what was their fate when they were retired.

Here's some images I gathered of them:
http://members.virtualtourist.com/m/p/m/1043ad/
http://www.flickr.com/photos/hubbers/414988292/
http://www.flickr.com/photos/23234983@N06/2221400785/
http://www.flickr.com/photos/riwayat/198410262/
http://www.flickr.com/photos/lemonlulondon/4641853765/

I also like seeing lots of different models used as taxi.
Here in Brussels it is often Mercedes, like in Germany. Many Skoda too now.
Some are much less common: I saw recently a Jaguar XF.

I guess the big challenge was to find a color scheme not being used by one of the cab companies. I didn't care for any of them. I remember the first time in DC in 1989-90 seing the different models used for cabs. I remember Pontiacs, Buicks, Olds, Mercurys, and smaller cars like Toyotas and Hondas being used as cabs. It was quite different than I would see in Philly or New York. It did add a little character to the city.

Yeah, those are pretty ugly!

Is D.C. the largest American city without a taxi medallion system?

All their choices are ugly as sin. I don't know if I'd want to see them all yellow, and if they were to go to a single color scheme, my first thought would be the Orange & Black ones they had years ago.

At least the proposed color schemes are a little colorful.
Not like here in Brussels where taxis are black with a little yellow on the side:
http://www.xpats.com/sites/default/files/taxi-web.jpg

They could do like in Italy: the taxis are all white. So then, in distance I can't distinguish them from municipal police cars.