What does complicated coinage have to do with distinguishing between colors? The Gemara is not about the complex laws of Niddah, but about the colors of blood.

And I don't think finding a unique "nature" to Bavel would help the "nature" explanation - Rebbi Zeira seems to be indicating he never inspects blood, not just while visiting Bavel.

the idea (according to Soncino) is that his inability to grasp that complex system demoralized him and stripped him of confidence that he would be able to grasp and apply the even more complex system.

the demarcation between one color (/ shade) or the other might well be considered a complex system.

But it isn't a complex system - it's recognizing colors. The Gemara never says he didn't answer questions about zava gedolah or poletes shichvas zera or any of the things that do make hilchos Niddah so complex that they were simplified into the 7 days across the boards system. It says this about colors. I don't see this as being a complex calculation to keep up with, as coin systems might be.

there is a reason that one needs shimush (under an existing expert) in seeing mar'ot dam. is this shade considered red or not? how about this shade? what if it has dried, is on this type of fabric (which provides a given texture or contrast). I can recognize colors, but I know that I don't know this complex system.

Would you say that someone who has trouble keeping track of, say, family trees would be unable to train in that shimush? It can be really hard to keep up with who is who. But what in the world does that have to do with seeing maaros? It does take expertise to recognize the fine distinctions between colors (according to some opinions, but some poskim write that colors are colors and all you need is a mesorah), but that is a completely different type of complexity!

see here about the complexity: yoatzot.org/question.php?id=4272 "A Rabbi spends years doing shimush (attending training sessions with his rabbi in order to learn which colors are acceptable in the tradition he carries."

Again, I'm not arguing that it isn't complex. I just think you are mixing apples and oranges. If I am bad at music, will I be bad at maaros? What about math? Glass blowing? I don't know why "bad at any given skill" == "bad at a skill of a completely different nature"

firstly, I disagree that it is an entirely different kind of complexity. Secondly, see how the gemara continues.

I saw how the gemara continues. You'll have to be more explicit if you want me to follow what you see. The Gemara has the story of Yalta talking about which colors she had previously shown and been told it was pure.

I just meant this part: כל שכן קאמר ומה רבה דידע בטבעא לא חזא דמא ואנא אחזי

which could make the relationship between the two not so critical

but the question really is, what was complicated about Babylonian coinage system

@joshwaxman No, it just makes one more complex than the other, on the same scale.

or, a measure of the mental agility to navigate the system

Rabba was a genius, say, in that he was able to navigate this complex system A (measured in vector A). And yet he didn't attempt complex system B, because he had the humility to recognize his limitations. So surely I, who was not able to navigate this complex system A, should not attempt complex system B.

@joshwaxman Was he OK with negaim and ohalos? Those are pretty complicated. The Gemara holds up tuma and tahara as an example of the most complex system to master, but R' Zeira chose maaros as the one that was too hard for him. I don't think he is just announcing that he is mentally weak.

Firstly, were negaim really something pratical in his days?

Since Rabba did not refrain from applying himself to negaim and ohalot, neither would have Rabbi Zera.

But aside from this, I think that the complexity of these systems are indeed along a similar vector.

It would not be if it was how many of coin A goes into coin B

But it would be if it was how color and size of coin A distinguishes it from color and size of coin B.

Among many different coins where are similar to one another.

To know for sure, one would need to study the Babylonian coinage system at the time of Rabba and Rabbi Zera.

I've gotta run, to pick up dinner.

@joshwaxman That's circular. Why would he not have refrained? Isn't it really complex, and he can't handle complex things? Why was it precisely this complexity that he couldn't handle?

@joshwaxman Enjoy - I'll be out for mincha. Thanks for taking the time to respond.

As an aside, Jastrow refers to a specific coin called a טיבעה