I've seen a few HPS lamps that have a part of their internal structure near the top painted or dyed green. Why?

Philips mogul base HPS lamps have green paint in side the dimples to indicate they are ALTO lamps.  Some others, in both medium and mogul base, have green cement in them.  I don't think there is any significance to the green cement, as I have seen both ALTO and non-ALTO lamps with it.