I was wondering what qualifies a server to be listed under the 'green' (preferred) servers?

I noticed
this post from a while ago mentioning some sort of trial system by which 2 servers a week can be temporarily 'greenified', hehe.

However, I haven't noticed this in action (I don't think!), so I was wondering if that system still stands? (Sorry I didn't want to bump up an old post so I thought I'd post this here!)
If not, what else can qualify a server to 'go green'?

Thanks in advance!