I think what you are seeing is gold-tin intermetallics.  The gold has dissolved completely into the tin of the solder, the solder has wetted out to scavenge the gold.  The gold-tin intermetallics have a high melting point, and when they've gotten to that edge that you see, there is no more tin, and the gold-tin IMCs have frozen up.  The color is a new one.  I found this regarding colored gold in Wikipedia.  Gold-tin IMC is usually just silver-colored, so I'm not sure where aluminum would have come from, or if there's some similar effect.
Colored golds can be classified to three groups:[2]
.the Au-Ag-Cu system, producing white, yellow, green and red golds; typically malleable alloys 
.the intermetallic compounds, producing blue and purple golds, as well as other colors. These are typically brittle but can be used as gems and inlays
Purple gold (also called amethyst gold and violet gold) is an alloy of gold and aluminum rich in gold-aluminium intermetallic (AuAl2). Gold content in AuAl2 is around 79% and can therefore be referred to as 18 karat gold. Purple gold is more brittle than other gold alloys, as it is an intermetallic compound instead of a malleable alloy, and a sharp blow may cause it to shatter.[7] It is therefore usually machined and faceted to be used as a "gem" in conventional jewelry rather than by itself. At lower content of gold, the material is composed of the intermetallic and an aluminium-rich solid solution phase. At higher content of gold, the gold-richer intermetallic AuAl forms; the purple color is preserved to about 15% of aluminium. At 88% of gold the material is composed of AuAl and changes color. (The actual composition of AuAl2 is closer to Al11Au6 as the sublattice is incompletely occupied.)[2]

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