My favorite place for lunch is, hands-down, Buenos Aires Verde. Their salads are AWESOME, and I love the hummus and seedy whole-grain bread they bring out as the cubierto. Good for dinner, too, unless you want a nice wine (their wine list is pretty lame.) They also do organic vegetable juices, thank god ... just never order the large jugo de remolacha. Please, just take my word for it
Artemisia is also lovely... haven't dined there in a while, but the last time I checked the menu, it seemed a lot more creative than it used to be. It's a beautiful space, with fresh ingredients and a professional staff.
Esquina de las Flores (the restaurant, not the grocery) SUCKS. My last trip there, the dishes were bland, skimpy, and overpriced; the bread was stale; and our waitress apologized for being slow, but she had a terrible flu (and YES, she was handling our food.) For the same price, you can get a far better vegetarian meal elsewhere.
There's also Granix. I can't put away enough in one sitting to make all-you-can-eat worth it, but if you're a big dude or just really hungry, it's a good option. You can put together any kind of salad you want, and the hot dishes are surprisingly tasty and creative.
P.S. What the heck is 'yellow food'?