What kind of discoloration the natural tendency for this particular tree is as the leaves age they get darker and more leathery. However other discolorations can mean many other things.

At the moment I can't describe the coloration because I pruned them all off a few days ago and can't fully remember. But darker and leathery is a very good description of the difference between the rest of the leaves that are lighter green and softer.

But the colorations goes beyond just a dark green, there is almost a grey to it if I remember, they stay that way for a long time and I guess longer than 6 months because I prune them before they fall off.

They have a tendency to get a light grey film across them that should disappear when u get them wet and come back when it dries. It looks to me like hard water residue but hasnt caused a prob for me. I dont normally trim mine off till ther is a new branch starting at the top of the leqf and havent had any fully die yet. However i keep new growth coming in low at all times by cutting the new nubs every time a new whip tries to grow out of the end of the old one. Im just sayin with all the new growth i keep inspiring at its young age the leaves dont last more than a few months at a time with the exception of a few attatched is a pic of mine

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pruning the discoloured leaves is probably the best bet. also make sure you give the plant adequate watering

Here is a link that might be useful: Bonsai Ginseng Blog