If i remember right, that post was "maybe X, maybe Y, or maybe even Z?", where "two yellow warnings" was Z. He also, if i remember correctly, said that even asingle extra trails (and thus centralization) would be very dangerous, and said that literally all of our centralization scores are yellow or red now. So while i think we've got 2 warnings now, i also think right now we're at the last yellow before red... So i think we shouldn't touch it until we finish the palace, unless we finally make sacred forest a free city and free up a tolerance.
I'll grab the quote in a second, but he's said we can build them for 2 econ and 2 art (and 1 main action), which is 1 econ/1 art "extra", compared to doing them now, when it costs 1 of each, and a main action.