Spectrum of Spider Webs

forwarded to sci.astro.amateur
From Bob Eklund of MWOA:

I'm looking for some technical guidance that would help me in a writing
project I'm involved with, regarding colors of the solar spectrum seen
in nature.

You may be familiar with the wheel-shaped webs produced by the banded
garden spider (AKA "orange spider.") Today I was looking at such a
web, stretched across the sidewalk above my head, when I noticed that
the web was reflecting sunlight, but pink and green only. The color
would shift from pink to green as I changed my position a few inches
backward and forward. I was looking more or less toward the sun, with
the sun perhaps 20 degrees to the side of the web.

My question is this: Is the effect I'm seeing here produced by
reflection, refraction, or a combination of the two (as in a rainbow);
or is it diffraction (as with a grating), or the thin-film effect (as
with an oil slick), or what? I'm tempted to think it's related to the
oil slick, only because the colors are the same. I have also seen this
pink-and-green effect in middle-level clouds made of water-droplets,
seen near the sun (not the same as the familiar halos seen in higher,
ice-crystal clouds).

My eyelashes, as well as scratches in my eyeglasses and windshield,
also produce somewhat similar effects, when I'm looking near the sun,
except that there I see a full spectrum, not just pink and green.

If anyone would care to enlighten me on this, I would much appreciate
it.

Thanks,
Bob Eklund
Chairman, Publications and Programs
Mount Wilson Observatory Association (MWOA)