Most ‘natural’ green insect pigments (in grasshoppers and plant bugs, for example) tend to fade, since they are chemically altered by light, and there is evidence that some are derived from chlorophyll eaten by the insects.
The green of the hairstreak, though, is not a pigment, but a metallic refraction effect caused by submicroscopic parallel grooves on the wing scales, which reflect only green light. Metallic green beetles use a similar mechanism.
In contrast, melanin (the default pigment across most animals) is highly stable, as are yellow and red pigments, which occur widely. There may be an evolutionary mechanism at work here.
Sedentary butterfly (and moth) larvae tend to eat green plants, and being all the same colour – as the caterpillars of many groups are – offers them camouflage.
But the day-flying adults need to combine bright colours (for mate recognition) with muted cryptic undersides (to hide or roost), so in this case green just may not be necessary.
