It seems like most Muslim countries have green in their flags (indeed, Qaddahfi's Libyan flag was just green!). This color seems to be associated with politically with most Islamic states, but I'm wondering why. Is there a source to the tradition? Is it in the texts? Or, is it just a coincidence?
It's the color of a lot of things in Paradise. The Quran doesn't really mention a lot of other colors positively, IIRC. So if someone wanted to pick out an "Islamic" color, it would just have to be green.
There is no relation between green and Islam. I think it just a coincidence. If you take a look at all countries' flags you will find that green is very common in many flags.
Qaddahfi's flag was green because he wrote a book called the green book, every thing in librya (before revolution) was named green. See Libyan flag history.
Also Saudi-Arabia flag is green, but I don't know why!
The old Islamic flag (at prophet Mohamed's time) was black with white writing. The color of Islamic empires after it differs in colors. Sometime black, sometimes red and sometimes green.