There are two ways to tackle this based on what you want:
Solution 1: Remove purple commits, preserving history (incase you want to roll back)
git revert -m 1 <SHA of merge>
-m 1
specifies which parent line to choose
Purple commits will still be there in history but since you have reverted, you will not see code from those commits.
Solution 2: Completely remove purple commits (disruptive change if repo is shared)
git rebase -i <SHA before branching out>
and delete (remove lines) corresponding to purple commits.
This would be less tricky if commits were not made after merge. Additional commits increase the chance of conflicts during revert/rebase
.