I remember asking one of my more scientifically minded buddies why this blue/green colouration sometimess occurs... And he said he didn't have a clue, but would find out.
Which he did, and apparently, it's all down to something called isoallin, a compound found in garlic, which breaks down and reacts with amino acids to produce a blue-green color. (He assured me that it's harmless, and doesn't affect the taste.)
Not sure where the amino acids come from (maybe the ginger?), but as Ian says above just chug in a glug of oil when you make your GG paste up and you'll have no such issues.
My last batch has been going for a week now and looks (and tastes) as good as it did the day I made it...
[EDIT]
Sorry Paul - we must have been composing posts synchronously...