Why do some smart-glasses displays use one color?

Information can survive without color coding

We would design status meaning around words, position and recognizable shapes first. Color can add emphasis when available, but a pending item should not become indistinguishable from a completed one when the display uses a single hue. This is useful interface discipline even outside monochrome glasses.

A card labeled "Waiting for approval" can remain clear without a yellow status dot. A completed item can say "Done" rather than relying only on green. The optical choice and the writing choice meet here: the interface needs to communicate its meaning through the output the hardware actually provides.