My 0.02¢ xref the drawings in question. This is what I did for my old job anything that was reference drawing (arch, sprinkler, civil) I would open their drawing change everything to 252 color and then xref into my drawing.
with plot styles 252 color was set to 50% screening. Meaning anything drawing by me would pop and the reference drawing was still seen but muted.
Only draw back is if you move project folders you will have to tell autocad where the xref files are located.