The 'visible page rank' is always three months plus out of date, and it's a 'snapshot' of the past, not necessarily a serial review.
So *POSSIBLE* reasons include:
- new-ish url
- Google unfriendly url
- page removed from index by Google
- page blocked by your instruction (eg noindex)
- page blocked by server or server error
(and I'm sure others can add to that list)
A two month old page can easily be in the index, but be greyed. Many 'deep' cms pages (eg second page in a forum thread), can be indexed but forever grayed by an unfriendly URL. An old page on a site with an unreliable or overloaded / limited server may be indexed now, but still grayed.
And I'm sure there's plenty of other 'greyed' pages that are indexed.
But while they ARE grayed, their 'link value' must be highly suspect at least!
In every case, it's important to look at the context; you know your site, and your seo and webmaster methods, so you can usually interpret a gray bar on your own pages.
If it comes as a surprise, then it's probably worth finding out if it's indexed or not :(
A gray bar on someone else's site may be telling you a lot ... or nothing at all.