This month is Gay and Lesbian Pride Month and Google is showing their pride by adding a rainbow to the search box for some search results.
For example, search in Google for [gay] or other related searches and you will see a rainbow on the right hand side of the search box.
Google also had this in 2008, but used a rainbow colored separated bar between the free and paid results.
Google is known as a company that is very gay and lesbian friendly. In fact, they came out with a controversial commercial about Gay rights.
That being said - it is Gay and Lesbian Pride month and Google is doing their part to bring it to light.
Forum discussion at Google Web Search Help.