Google rolled out a new design to its Android Messages app quite recently, and soon after that, it reverted the app to the old design, but the company just fixed that, as the new design seems to be back. Now, along with a new design, Google also included a 'Dark Mode', which was also removed soon after it was released, but it is now back as well. The new design and dark mode reappeared in the v3.5.052 version of Android Messages which will hit your device soon, if it hasn't already.
Now, Google reverted to the old design via a server-side change, and the reason behind it may be some issues that apps like Twitter and Nova Launcher encountered when the night mode was enabled. It's possible that both Twitter and Nova Launcher's developer amended that issue, or perhaps Google changed a part of the code in Android Messages in order to avoid such issues. Whatever the case may be, the new design is now back along with the dark mode, whether you like it or not. The new design of Android Messages is here to complement Android 9 Pie's design, as Google changed things quite a bit in the new version of Android, as part of next-gen Material Design, as Google refers to it.
So, essentially, you'll get a bit less color in the new design, as Google removed background for the top bar in Android Messages, and same can be said for the colored notification bar as well, they'll all be white-colored when you install this new update. If you'd like to start a new chat / send a new SMS message, you'll need to hit the 'Start chat' button in the bottom-right corner (or a contact directly), instead of the '+' button which was available thus far. Such changes are visible in other submenus as well, and whether you like it or not, this is what Android 9 Pie is aiming for, so such a change was to be expected. Other Google-made apps will probably adapt in the near future as well, and the same will happen to a ton of third-party apps once Android 9 Pie gets more traction.