On a good day, the tiny tortilla factory that anchors Sobre Masa in Brooklyn churns out 5,000 tortillas, in shades of red, yellow, white, perhaps blue. The colors that day—and, in turn, the distinct flavors, aromas, and textures—depend on the availability of the heirloom corn they buy from small Mexican farms. “We’re fourth in line, basically,” said chef Zack Wangeman, after the farmers save enough for their families, livestock, and next year’s crop. He has to explain to clients, “Hey, you might not know what you’re gonna get—but that’s a good thing.” Zack and his wife, Diana, both Oaxaca natives, opened the all-in-one tortilleria, cafe, retail shop, and taqueria and bar last October; it’s their second project after a now-closed restaurant by the same name. The tortilleria is the first in the city to use only imported heirloom Mexican corn, sourced through Mexico City-based purveyor Tamoa. “We’re a corn-driven establishment …