Split pea soup ?
That's what I thought of.
I just thought of some cauldron with a green potion.
My first thought was palak paneer, which is neither a soup nor from Greece. Are there any famous green soups in Greece?
No 'famous' green soups but you can always make zucchini/courgette soup and it will be green. Someone suggested spinach, I don't know if that's possible, but sure, why not?