I have a normal clear glazed (white) porcelain mogul socket, but I also have a yellow one. I have also commonly seen pink and gray ones floating around on Ebay. Why would they go through the effort of putting colored glaze on the sockets? What benefit does it serve? I thought maybe it could be an indicator of PSMH or HPS, but I see them used in situations where that doesn't make sense. Any thoughts?

Edit: Now that I think about it why glaze them at all? It it even necessary?

The glazing on insulators makes the surface smooth, so it is less tendency to hold moisture and contamination on its surface, so less risk of a flashover.
What the different colors mean, if anything at all, I have no idea.

As far as I know, the pink E39 base lampholders are designed for use with open rated quartz metal halide lamps with the proprietary EX39 base and have a structure that prevents standard E39 base lamps from running while the yellow E39 lamp sockets have a notch that allows the use of horizontal burning quartz metal halide lamps that use a locating pin on the base to prevent accidental installation in regular E39 base fixtures. However, you can use standard E39 base lamps in the yellow E39 lampholders just fine. Lastly, the white E39 lampholders are the standard lampholders.