Former Google Search Quality Analyst, Pedro Dias, posted a screen shot of a Google top bar test on Google+. The test is that Google has replaced the black top bar with a gray black bar.
Here is a picture from Pedro:

I am not able to reproduce this but we know Google does tons and tons of experiments from algorithmic experiments to user interface experiments and more.
Personally, I kind of like the light gray look. It is more relaxing and less in your face.
The original Google black bar was introduced about a year and half ago, so by Google's standards, it is time for a change.
Forum discussion at Google+.

Comments:
Marek Płatek
01/22/2013 01:50 pm
I saw it a couple days ago and looks much better https://plus.google.com/109057976580491900523/posts/aDpNZyZSzEA :)
Ninet Hosting
01/22/2013 01:58 pm
They are making changes to often.
Ryland Bacorn
01/22/2013 03:58 pm
I want a grey bar. But what will it do though?
Sergei Lazarev
01/23/2013 04:08 pm
Got this 2 days go. Looks better than the current one.