In the iOS app's revisions page all revisions except initial have yellow background for description:
What is the reason to make them different?
- App Version: 126.96.36.199
- Device: iPad Air 2 (Wi-Fi)
- OS Version: Version 10.3.1 (Build 14E304)
This will be removed in 188.8.131.52.
There are two types of revisions "single_user" which corresponds to an edit, and "vote_based" which corresponds to events like bounties, being featured on Twitter, etc.
When this view was designed 3 years ago, the web had a yellow backgrounds for "single_user" which is why the app does. At some point the web got rid of it but I never noticed. As for why there's no background on the initial revision, that's because there's no comment and it looked weird in the app so I gave it one, and I felt white was the most appropriate.
I'm getting rid of it now.