Scales on the ventral side of swordtail butterfly wings enhance blue/green coloring via light reflection and diffusion.
“The wings of the swordtail butterfly Graphium sarpedon nipponum
contain the bile sarpedobilin, which causes blue/green
colored wing patches. Locally the bile pigment is combined with
the strongly blue-absorbing lutein, resulting in green
wing patches and thus improving camouflage. In the dorsal forewings,
the colored patches lack the usual wing scales, but instead
have bristles. We have found that on the ventral side most of
these patches have very transparent scales that enhance, by
reflection, the wing coloration when illuminated from the dorsal
side. These glass scales furthermore create a strongly polarized
iridescence when illuminated by obliquely incident light
from the ventral side, presumably for intraspecific signaling. A
few ventral forewing patches have diffusely scattering, white scales
that also enhance the blue/green wing coloration when observed
from the dorsal side.” (Stavenga et al. 2010:1731)