The colour is due to diffraction of light by fine grooves in the scales on the wing. The structures are actually rather complex but there is a good description at this website
http://www.microscopyu.com/articles/pol ... intro.html To see the colour, the incident light (from, say, the sun) has to strike the wing at a particular angle AND the observer has to view the wing at a particular angle. These angles have to be the same for both wings but you don't necessarily have to be directly above the butterfly as these pics (from Bernwood in Oxfordshire) show. They were taken quite late in the afternoon, when the sun was getting lower, so this may also have lowered the optimum viewing angle.
![PE1_Bernwood.jpg (119.47 KiB) Viewed 1694 times Bernwood Forest, Oxon - 19th July 2012<br />Nikon D300s with Tamron 90mm macro lens - 1/350s@f/6.7 ISO800](./files/thumb_11310_7c5191fd31eb34d2742144636fdb857a)
- Bernwood Forest, Oxon - 19th July 2012
Nikon D300s with Tamron 90mm macro lens - 1/350s@f/6.7 ISO800
![PE2_Bernwood].jpg (146.78 KiB) Viewed 1694 times Bernwood Forest, Oxon - 19th July 2012<br />Nikon D300s with Tamron 90mm macro lens - 1/180s@f/9.5 ISO800](./files/thumb_11310_dbca065da5797b8aaa55c2a99e4bf443)
- Bernwood Forest, Oxon - 19th July 2012
Nikon D300s with Tamron 90mm macro lens - 1/180s@f/9.5 ISO800
I think your best chance is to stay close to the centreline of the butterfly, so that the angles of the light rays are the same from both wings. If you move too far to the side, it will only be possible to see the purple on one wing
Mike