The last time we discussed the grey PageRank Google toolbar indicator was just about two years ago. It was then when the gray/grey color changed from meaning banned to having no clear meaning. Back then, there was a blank white bar and a blank gray bar, both with PR0s. White meant PageRank was not yet calculated for the page, gray likely meant a ban or penalty. This is no longer the case.
Here is a picture of gray:
Today, we have a thread at WebmasterWorld discussing what the gray bar means. There are a few theories that I will share with you.
One person thinks that gray is given to pages with not enough PageRank to reach the 1 level. Another person thinks that pages in the supplemental index receives gray. Some believe that pages with penalties still receive the gray indicator. Let me quote Tedster's post on this topic:
It is no longer true that any url gets the TBPR that it earns. Some types of pages (lists of links for example) seem to get flagged as an automaic gray bar - and that's that. I used to think this was manual, and in the beginning it may have been. But now I think that original "seed set" has been used to generate a machine-learning algo that is intended to locate other pages of the same nature. This approach to various segmants of the algo is something that is more and more in use. All those PhD in Statistics folks need something to do, right?
For a while, my theoretical algorithm was misfiring quite a bit and the mysterious graybar disease was nearly epidemic. Just this past month, I'm seeing some of those peculiar gray bars come back to white or even green. That could indicate either that the machine-learning has improved, or something about its logic has been tweaked.
Can it be a form of artificial PageRank, is it Google classifying certain pages in a group, is it pages in the supplemental index or is it just nothing?
I wonder if we can get a response from Google on this, knowing that we should not live and die by what the PageRank indicator says in the Google Toolbar.
Forum discussion at WebmasterWorld.