I don’t have too many details surrounding the creation of these parts, but perhaps i can explain what’s going on by looking at some detailed pictures.
Here we see the left shoulder pad. It is built out of .9mil ABS. It was primed and painted with several varying colors of paint than a vinyl decal was added. I than weathered the plate by using an angle grinder and some sandpaper.
The same was done with the above plate, just with a different design.
Here we can see the detail on the weathering used on the left chest plate. It was painted in a similar pattern as the shoulder plates. Multiple colors followed up by a heavy weathering technique which, in this case ONLY included an angle grinder. You can imagine which angles where used to hit the plates briefly or for a long time to achieve the resulting details.
And as a happy accident i happened to notice the armor showed the detail exaggerated under a blacklight, so perhaps this can help as well as a guide.