As with just about anything else that Google puts out, the Play Store has gone through quite a few updates visually, and just recently they started testing a new UI for it. The start of these changes began appearing back on October 26th, and now it looks like some users are actually starting to see these changes come through on their devices. Although, it is worth noting that the changes in question do not seem to be rolling out to very many users at the moment, which suggests that this is more of a small test instead of the actual push of the changes to the public.
These visual touches are not major sweeping changes or any sort of a complete overhaul, but they are different enough that users will take notice. Included in these changes is a new more slender looking install button for content. Upon opening the dedicated page for any app or game, the button for the install no longer takes refuge to the right side of the screen just under the icon at the top of the page. Instead, it spans the length of the page, and below it sit slightly different looking icons for the download numbers and the star rating.
While the new install button will certainly be noticeable, the first thing that uses will see once the UI has been updated is the tweaks Google made to the main screen of the Play Store upon opening it. The green is darker than before and is now matching the color used for Hangouts, and the app carousel that used to sit in the header with the search bar is now below the search bar. In addition to the new placement, Google has also given each application that sits in the scrollable list of featured apps a visual touch up by using a card-style layout instead of banner image that takes up the length of the header like it used to. Some of the visual updates in the Play Store seem to be appearing for some users in India, so Google's test is likely a regional thing, although there's no indication that only users in India are seeing the changes. There's also no way of knowing when Google will roll these changes out to all uses.