The red line is a sodium D-line emission, that is present in high-pressure sodium lamps. Sodium lamps that have even higher pressures can also contain blue-violet lines that are mainly due to mercury. In most sodium lamps you will only see the yellow doublet but your lamp could very well have these lines.
I think I'm doing the same experiment as OP and the lab manual states that the sodium D-line is the yellow line which is actually a doublet. I'm not sure what the red line is either as it was too faint to get an accurate measurement.
As for Vanadium 50's hint, it seems that he's implying that the line is caused by impurities in the lamp. That's what I take from the hint anyway.