I suspect having a color coding material/technique that can apply to random bits of random plane parts, that won't get rubbed off / broken off / obscured by dirt/grease/grime/etc, consistently throughout the plane, is probably harder than it appears at first glance.

Well, one less part on parts list assuming they are same length

> and how this wasn't an issue in manufacturing?

Manufacturing does same exact thing every single time in order and then someone checks it.

But this was something staff don't do often.