Today I ordered a green tea in a restaurant, in Switzerland (Graubünden), saying "und grüne Tee". The waitress repeated my order and it sounded to me like "grün Tee". Now I'm looking at a declension table and I figure I should have said "grünen". What was it that I heard from the waitress?
She said "Grüntee". It's one word, just like "Rotkohl" or "Blaubeere". "Grüner Tee" is very uncommon in Switzerland.
Oftentimes the -en ending in DE-German (and I must say that I'm not sure whether the same is true for Switzerland or Austria) is somewhat "hidden" and when pronounced may sound like grün'n Tee. While this normally doesn't cause problems (gut'n Tag), it causes you to hear grün when somebody actually says grün'n. So you might just have misheard what she way saying.
As stated by dusky, other German speaking countries use words like Schwarztee or Grüntee instead of saying schwarzer Tee or grüner Tee, too.