Maybe do a multicoloured coin? But out of the options listed I would go with dark blue.
I have zero artistic talent, so if you want something other than a simple color, you'll need to create the image.
If you create an image, you can modify more than just that half-coin. I did that originally only because more than a half coin's worth of color is distractingly colorful. Colorizing half a coin doesn't symbolize anything. If you create an image, make sure it's not more distracting than the normal colored half-coin.
If both coins are next to each other, I think it would be easy to spot the difference...
But if they're not, different shades of green or purple/violet could look the same.
Yeah, that's why I chose yellow originally. All of the other primary colors are taken. But it seems that people don't like yellow, so I'll need to reuse some color. Normal blue is used for global moderators and very light blue is used for local moderators, so this dark blue is also a reuse, though maybe it's easier to distinguish.