The green of a terminal is not a design choice. It is a side effect of a phosphor called P1, which was selected in the 1950s because it had good persistence — the glow held long enough for the eye to read it before the electron beam came back around. The alternatives at the time were white, which faded too quickly, and amber, which came later.

By the time better phosphors existed, green had become the colour of computing. Not because anyone decided it should be, but because enough people had used it for long enough that the association stuck. The colour survived the reason.

This is a pattern that repeats in technology more often than anyone admits. A constraint produces a convention, the constraint is removed, and the convention stays because it has become a preference.