They mainly use blue, but if I recall correctly, when they first launched the new jw.org website, it was a green logo.
I think they went to blue for the umbrella brand and reserved the alternate colors for different products: JW Language app is now green, JW Library app is Purple, etc.
See: The Wayback Machine Capture right after the launch with Green as the primary color.
https://web.archive.org/web/20130701070835/http://www.jw.org/en/