The Flag Bulletin No. 155 (1993) carried a short note on the Ecology flag.
Here are the main points:
The theta/ecology symbol was made by Ron Cobb (a cartoonist) and published for the first time on 25 October 1969. The symbol is a combination of the letters e and o, from the words environment and organism. The association with the Greek letter, and the word thanatos (death) was made by a magazine called Look on 21 April 1970. Apparently, this magazine was instrumental in spreading the symbol and making it into a flag.
In the 21 April 1970 issue of Look the theta/ecology symbol was put into a U.S. style flag. In this first version the symbol was colored chartreuse, according to the Flag Bulletin. I must admit that doesn't tell me much. The dictionary defines chartreuse as "a color varying from a clear yellowish-green to a strong greenish-yellow." Later, the color is usually white. Other versions than the green-white flag are also known, according to the Flag Bulletin.
The Flag Bulletin also mentions the first ecology flag, made by the San Francisco based Paramount Flag Co. in August 1967. This flag was also based on the Stars and Stripes. Dark green was used instead of blue and light green instead of red. The colors represented "pure air and green land" and environmental action.
I suspect that this ecology flag is almost exclusively a flag used in the USA. I have never seen it used in Norway.