How should I interpret the results of a lead paint test?
The details for the 3M LeadCheck product read:  


  turning bright pink when lead is present  


I don't see any "bright pink" in your photo, so it seems that there isn't a significant amount of lead present.
Your test kit should contain a "Test confirmation card" which you can use to compare your results with their examples.  

Even if it did indicate some lead though - if you have to tear the walls apart to get to it, then it doesn't matter.  Lead in paint is harmful when ingested, so unless you or someone you know is planning to eat your walls, you'll be fine.