Mr. Bloomquist,

to my oppinion your observed varying colours (gold and especially
purple) are pointing to an oxide layer on the solder. Oxides are
dielectric and therefore are mostly transparent for light. Also oxide
layers can be very thin (less than one micrometer). Such thin
transparent films on reflecting surfaces (like the tin underneath)
produce light interference phenomena which will result in coloured
reflected light. The same phenomenon produces those colours you will see
when your are looking on an thin oil film over water or when you are
looking at heated iron after cooling down. 

To prove wether you got thin transparent oxide films on your solder
surface, you can do two simple things:

1.) Look at the surface under different angles, lets say nearly parallel
to one point of the surface and nearly perpendicular. Best suited are
planar or only slightly curved surfaces or parts of the surface. If
colours are changing, you got this interference phenomenon.

2). Use monochrome light (blue light should be most useful because of
its short wavelength). Use some glass filters for producing the blue
light. Look at the surface at different observation angles. Again a flat
surface is best suited. If it is interference, at some angles the
surface should appear to be black.

You probably have to have some patience to see these effects. Especially
you have to carefully select an appropriate, not to much curved surface
(nearly flat). In both cases, be sure to use a point light source (like
a bulb), which is sufficient far away (lets say at least two or three
meters). This bulb must be the only light source in the room!

Hope this will help a little. I would be interested, if I am right. 

Ulrich Korndoerfer

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