I guess, to use the terms of your analogy, I don't think people disagree on what blue is. "Don't add backdoors to e2e encryption" is blue; and plenty of people who are coded all over the political/ideological spectrum recognize it as blue and want the wall to be blue.

You seem to be saying that people can't paint together unless everyone agrees on who holds the brush, what brand of brush is used, and what everyone's broader philosophy of painting is.