color - Origin of "big green button"?

I found meme about big green button.

When people visit download sites (like software directories, file hosting sites, torrents, etc) after completing reading page title and deciding to download they look for big green button...

I have interest to know origin of color and size choice.

I found this sources:

Also possibly related links:

UPDATE. Interesting color marking agreements can be found at: http://en.wikipedia.org/wiki/Triage