Microsoft has shared the changelog for this month's set of updates for Office Insiders on Android, which is build 16.0.13801.20004. This month's updates only seem to target the Outlook app, but there are a handful of interesting additions.
Perhaps the most notable change is the addition of different colored themes for the app. Until now, Outlook has only ever been available in its typical blue color, though you could choose between a dark and light theme. Now, in addition to that, you can choose from a set of different colors to apply to the colored areas of the app.
Another obvious change will be that the search feature has been moved to the top of the app, instead of being its own tab at the bottom. The old search button has been replaced with a new button that looks similar to the Actions button on the Office mobile app, though Microsoft doesn't acknowledge it or its use in Outlook. We haven't been able to get the new design just yet, so we're unable to verify what the new icon is for at the time of writing.
Additionally, Outlook is getting the ability to make Teams calls using voice, though this is only available for users in the U.S. with the language set to English. There's also a new button to create a task in Microsoft To Do from an email, found in the three-dot menu in the top right corner. Finally, the app can now sync your contacts from Outlook on the web, Windows, or Mac, including subfolders, by assigning labels and categories to each folder.
These features tend to roll out gradually, so it's possible that you can't use many of these just yet. Over time, they should be available to everyone. Also, keep in mind that while Outlook features are listed under new Office builds, the Outlook app itself uses different version numbering, with the latest version being 4.1.2103.0, as of the time of writing.