Enabling and disabling
NSConstraintBasedLayoutVisualizeMutuallyExclusiveConstraints is safe. This feature allows developers to debug their application's layout problems.
Autolayout and Constraints
Constraint based layout is a mechanism Apple recommends developers use to craft their user interfaces. The mechanism is often called Autolayout in developer documentation.
Autolayout is tough to get right. It is possible to have constraints that conflict. Apple have added numerous features to help flag these conflicts and to help developer's fix them.
NSConstraintBasedLayoutVisualizeMutuallyExclusiveConstraints flag is one such tool to aid developers.
Not Intended for Users
I presume you did not enable
NSConstraintBasedLayoutVisualizeMutuallyExclusiveConstraints yourself. Preassuming this, it suggests Photos.app's engineers have an autolayout bug or that some other process has enabled this flag on Photos.app.
Either way, the purple interface was rightly concerning as a user. No user should ever see, or need, this debugging tool. Somewhere a mistake was made or a bug exists within OS X.
Safe to Disable
The suggested solution in the discussion thread deletes any value associated with
NSConstraintBasedLayoutVisualizeMutuallyExclusiveConstraints. This is safe:
defaults delete com.apple.Photos NSConstraintBasedLayoutVisualizeMutuallyExclusiveConstraints