It appears that the Gray Google logo in the search bar might be turning into a rainbow soon. According to Android Police, a few of their readers are reporting a multicolored Google logo. Basically, if you're using the Google Experience Launcher on Android 5.0 or later and tap on the Recents button on the right side, you should see a google search bar with a multicolored Google logo, instead of the gray one we normally see. If you're on Lollipop and using a third-party launcher, you'll likely only see it if you add the Google Search bar widget to your home screen. We've attempted to replicate the change on our Nexus 6 and Nexus 9 to no avail.
It's looking like this change on the Google Search widget is actually a server side change. According to Android Police, they are getting tips from users on various versions of Google Play Services with the update. So it appears that Google is hitting the switch for a few people at a time. Looks like another slow rollout. It's not a huge change, and I'm willing to bet that most people probably won't even notice it. But it is a change. It sorta brings their experience in line from mobile to desktop, as the Google search on desktop is multicolored and currently, the Google search bar on Android is not. Unless you have gotten this change already.
Google might be doing some testing to see how users react to the change. As it appears that only a small number of people have gotten this change on their devices. If that's the case, we may or may not see this change hit other devices soon. With Google I/O coming up in a little under a month, we're likely to see a lot of testing from Google and a lot of rumors and leaks coming out ahead of Google I/O which happens every year.
So far, it appears that nothing else has changed. Everything looks and acts the same, Google has just changed the color of their logo. And that's about it. So not a huge change, and not something that you should get upset over.