I have both; and olive M12/UM84 that I attached to a Tactical Tailor shoulder rig, and a black UM84R.
The color choice is only as relevant as a choice/concern for you individually. As a "field" color, the olive might be marginally better, but I wouldn't be overly concerned about a black one. The interiors as I recall are a fabric (or fabric-like) weave. Probably easier on a blued gun than kydex, but might impart more wear than leather or horsehide, particularly over time/use.
I'd call Safariland/Bianchi customer support and ask specifically about the holster body and flap colors on the olive ones; their CS tends to be very good and responsive.
Best, Jon