This white Google logo was also on Google Poland a few days ago.
The reason it is mysterious is that the alternative text on the image has no clue on what the white logo represents, nor does the logo hyperlink to anywhere, plus the image name reveals nothing interesting.
A Google Blogoscoped Forum thread speculated the white logo on Google Poland was a form of a memorial for certain victims. But if that is true, why is Google Hong Kong showing the special logo - I don't think there is a public memorial today in Hong Kong. Some are speculating due to the earthquake in Qinghai, China but why not link to this or something to represent what this is about.
Hat tip to Michel for pointing these out to me.
Update: Google sent me this statement on these white logos:
From time to time we show a special colorless logo on our homepage in recognition of a current loss or tragedy. Users in Poland recently saw this muted logo, and now we're showing the logo to users in China and Hong Kong to pay our respects to the victims of the Qinghai Earthquake. We are saddened by the loss of life and great damage caused by this natural disaster, and our thoughts are with the people of Qinghai, China.
Google also confirmed that the first time they used the white or colorless logo was last week on Google Poland.
It is my understanding that Google uses this logo only when a government body declares a state of mourning.
Update: Baidu also has a colorless logo: