Is purple primer turned yellow still good?

I assume this is part of the plastic pipe glue.

I wouldn't chance it... I'd buy fresh glue... and primer if it's that old... you won't have as much chance of a poorly joined pipe causing problems in the future.

The reason primer is colored purple is so that the inspector can see that you have primed the joint before gluing it. For inside joints that can be seen we use a clear primer. Toss the old primer away and purchase new. Good luck, Tom

Replace the primer and pipe cement. If your primer has been sitting long enough to turn it's pretty purple color to yellow you can bet the pipe cement is history.