I am wondering if this is a bug with dart fix itself, cc @bwilkerson
As I understand it, there is a rule to fix backgroundColor, and another rule to fix errorColor, but when backgroundColor and errorColor are both being used, only one of the rules is being applied.
I am going to see if I can reproduce this tomorrow in a smaller test case, and if so I'll file a bug and link it back here. I'll follow up tomorrow.

It's certainly possible that there's a bug, but I think there's a different reason for what you're seeing.

When the transforms (the things with titles) in the fix_data.yaml file are applied, they are applied sequentially, one at a time. That is, each transform is independent of the others.

When the transform for errorColor runs it produces the edits you're seeing. When the transform for backgroundColor runs, it produces a similar set of edits, but those edits aren't applied because the framework detects that they conflict with the first set of edits (from the errorColor transform). In order to prevent user code from being corrupted, the framework discards the second set of edits and then continues looking for other problems to fix.

I recently (11/23) updated dart fix so that it now runs in a loop:

compute the edits for as many fixes as possible
while (there are edits to be applied) {
  apply the edits to the files
  re-analyze all the changed files
  compute the edits for as many fixes as possible
}

I'm guessing that this change hasn't yet rolled from the Dart SDK into Flutter. When it does I believe that it ought to fix this problem.

We could confirm that hypothesis by taking the output from the first run of dart fix and run dart fix over it again (manually iterate). If the edits for backgroundColor are applied on the second run, then the change ought to solve the problem.

Thank you very much @bwilkerson! That makes a lot of sense.
@guidezpl it looks lie this has not been synced with master since that change was made, can you update and we can see if this is already fixed? :)

@guidezpl I was actually looking into deprecating errorColor myself, do you plan to proceed with this PR?

FYI #87868

Thanks @bwilkerson and @Piinks! I tried it out and it did indeed apply both fixes. However (expectedly), it creates an additional copyWith. I wonder if this can this be improved?

  // Changes mades in https://github.com/flutter/flutter/pull/94156
  ThemeData themeData = ThemeData();
  themeData = ThemeData(colorScheme: ColorScheme.fromSwatch().copyWith(error: Colors.red).copyWith(background: Colors.grey));
  themeData = ThemeData.raw(colorScheme: ColorScheme.fromSwatch().copyWith(error: Colors.red).copyWith(background: Colors.grey));
  themeData = themeData.copyWith(colorScheme: ColorScheme.fromSwatch().copyWith(error: Colors.red).copyWith(background: Colors.grey));

@markusaksli-nc yes, I'll let #95870 land first and then get this one in.

No cycles to finish this at the moment, will get to it eventually.

Can't get the new commits to appear, will close and reopen a new pull request: #110162