I think Marx had near-black irises, both from looking at photos of him and descriptions from the time like this:

His physique is well knit, massive, erect. He has the head of a man of intellect, and the features of a cultivated Jew. His hair and beard are long, and iron-gray in color. His eyes are glittering black, shaded by a pair of bushy eyebrows.

Obviously not endorsing the physiognomy there.

Edit: Awesome portrait, just wanted to mention since you were discussing eye color.