Metallic sheen is an interesting phenomenon. It has to do with alignment of particles all in one plane, reflecting back light to the eye. If you think of when this occurs in other situations, it will be simpler to understand. Think of a thin layer of gasoline or oil on water viewed at the right angle in sunlight; Or... layers of mica viewed at the right angle; Or raku glazing on pottery. Now it is possible to create the same excessive effect in watercolour painting when too much pigment is used with not enough water. This is a situation where the mass tone colour is excessive and a metallic sheen begins to occur, (too much paint, not enough water).

In the case of pysanka dyeing, I think it arises when heavy concentrations of dye are used, compared to amount of water. It also occurs when eggs dyeing sessions are overly long, allowing for a mass-tone situation to occur on the eggshell. The individual dye particles (which are smaller than the ones in watercolour pigments) are no longer clinging to the irregular surface of the shell (scattering light in many directions). Instead they begin to build up to the point where they are all laying in a flat thick coating on the 

shell. At this point, they are aligned like mica plates in one flat layer and seem much more likely to reflect back light to the eye. This seems most often to occur in dark colour situations, because we want to achieve heavy dark uniform dyeings. May also be due to high concentrations of dye particles allowed by the manufacturer or just the shapes of those particular molecules in general.

......This sheen disappears when something causes the surface to no longer reflect the light in the same exact way. Just coating the shell with the protective finish seems to do this. Or successive dyeings that wash off the overload of particles will cause it to disappear. Some people think it is neat and wish it would stay. I am not sure how to preserve this state. Even handling the egg later will cause the sheen to disappear. 

Metallic or Interference paints are a bit different. They are designed to do this alignment (more readily and on purpose). They usually have small particles of mica ground into their mixture. They also have a "carrier" that will hold the particles in positions and fuse them in place. Since mica is a rock and basically insoluble, it settles out of suspension quite easily so a carrier is needed to keep them suspended. I do not know the composition of the gold dyes used in pysanky. However since I do know they are derived from dyes used to colour fabric, it is quite possible that they contain some amounts of mica. Mica may be part of the insoluble substances that often settle to the bottom of the gold dye jars, or hang like clumps on dyed surfaces, much to ones dismay. Since our dyes are made soluble with only water and vinegar there is no real carrier to hold heavier particles in suspension.