It seems that Google wants to avoid any type of confusion about when you are using normal SMS/MMS chats or RCS chats in its Messages app, since they will reportedly send a special background wallpaper for the latter.
A special background wallpaper could reach Google Messages RCS chats
As of today, Google Messages normal chats and RCS chats have the same background, which could cause confusion among those who are not sure how to differentiate between the two. So, to solve this problem, the company is adding background wallpaper to Google Messages RCS chats as an additional indicator.
The new background consists of overlapping bubbles whose colors appear to be taken from the Android Dynamic Color system. In its current state, the background wallpaper is quite noticeable when you use light theme, but it is much more difficult to distinguish when using dark theme, as spotted by 9to5Google:

The feature is still in development and its rollout has not yet started, so there will probably be improvements in more advanced stages. At the moment, it is not known how customizable the background wallpaper will be, but the source claims that it is a separate customization from the Custom Bubbles that are also in development.
So, in the future, in addition to the prompt in the text field and the indicator in public avatars, users will be able to differentiate Google Messages RCS chats thanks to the new background wallpapers. This method seems the most visually explicit of the three, and will be especially useful for those less technologically savvy.
It is not yet known when the rollout of the feature will begin, but there are still things to improve, like the aforementioned issue where the background wallpaper is very discreet when using dark mode. Let’s hope that when it is ready it will offer multiple customization possibilities for chat backgrounds too, like other popular messaging apps do.