It appears gold (Au) from the leads is diffusing into the solder and forming a solder-gold intermetallic compound (IMC), which is indicated by the telltale purple color on the surface.

According to published reports, a 3-5 wt. % of Au in solder can cause embrittlement of the solder, so you will probably want to look into adjusting your gold-coated leads, your solder material, and/or your reflow process (such as time, temperature, etc.) in order to minimize or eliminate this problem.

Otherwise, you risk brittle failure of the soldered joints over time and temperature cycles.

Here are a couple of articles on the Web that discuss Au (gold) IMCs:

(1) Cambridge University Press

(2) Gold Bulletin 

I'm not certain but it looks like a Gold / Tin intermetallic. The percentage of gold in a joint should be less than 3% otherwise you could have a brittleness problem.

If you Google "Gold Tin Intermetallic" you will get lots of hits on the subject.

Can the discoloration be removed by cleaning with a solvent (such as acetone), i.e. dip a 'Q-tip' in acetone and try to rub off the discolored area.

Since gold is essentially chemically unreactive in ordinary environments, my suspicion is that the discoloration is coming from an as yet unknown source. For example, trace amounts of plasticizers can be leached out of plastic tubing when such tubing is used to transport nitrogen gases from tanks and connected to instruments or chambers.

I need a little more information, but this kind of 'systems analysis' will disclose the problem.

First, try to get a trace analysis as to the composition of the discoloration itself. An IR microscope may be the ideal instrument, especially if the discoloration cannot be readily cleaned off.

After some thought, I have submitted this second answer.

There has been much discussion lately that "no clean" is a misnomer. In fact, cleaning is generally still desireable ... In this case, a little more detective work (i.e. identify the composition of the discoloration) should provide the information needed to isolate the root cause and eliminate the problem.

Also, how good is the gold plating? Trace amounts of other more active metals can result in strange chemical effects.