I had this EXACT problem with my LG RT 50PZ60. Basically developed after about 8 months. Only occured when the screen was cold....and iot disappeared after 10 minutes. Exactly the same red green sparkles in dark scenes. If the screen was switched off then on within say an hour few sparkles were visible. If the screen was off for a few hours the problem was at its worst.
Anyway I rang LG customer support who advised me my nearest service centre....Gladesville Electronic Services. They said they had seen this before and it just needed adjustment. Anyway 2 weeks later they said it could not be adjusted anymore, so they would ask LG for a new panel.
LG no longer made the panel so offered me the new 50 inch LG. Anyway, I asked for my money back they gave me a store credit and I am now the proud and happy owner of a Pioneer 505. No more solarisation, no more sparkles.
I spoke to a girl called Teneille at LG customer service.
I have to say LG was pretty good. I didn't rant about it, but merely said no I didn't want the new LG, could I have my money back and they said sure no probs. My friend ordered the same LG at the same time. He returned his after 3 months with a different problem, and also got his money back and now has the pioneer also.
Good luck with your dealings.