... in sheet web on a tree trunk.
The orb web is horizontal and lit by sunlight in one area only. The colours are due to forward scattering (the sun was directly in front of the camera and about 10 degrees above the plane of the web. Typical sticky horizontal strands of web are covered with a glue that collects in tiny droplets some 5-10 microns in diameter. Wavelength dependent interference and diffraction as the scattered sunlight recombines produces the colours: more pronounced in slightly out of focus images as here. Colours in one particular direction vary as the spacings of semi-regular lines of droplets vary along the threads.
I have taken the liberty of enhancing and manipulating (with a motion blur etc.) a small part of an image of a dandelion head against the setting sun taken by Conall. The original image is here ... www.flickr.com/photos/conall/26451858144 .... The same irregular bands of colour that occur on spider webs are present. I wonder why exactly?
The parachute hairs of a milkweed seed (upper left) lead to circles of light as sunlight is scattered from hairs oriented along tangents to circles centred on the sun. The sun circles show the colours seen in spider webs by a process of diffraction about sharp edges and interference. The hairs of dandelion seed parachutes show the same effect.
Colour returned from sticky spider web in white LED light. The effect is somewhat similar to sunlight speckle and coloured sun circles what also require partial coherence. The radial foundation lines and the centre of the web that lacks the sticky droplets return no light and appear invisible. The spider is overexposed. The image was taken at night using the led light on the camera as the only significant illumination.