random part transparency colour

Go to Options and uncheck the "Random Works" option

Seriously, I have experienced the surfaces of cosmetic threads will turn opaque in assemblies. These particular parts had transparency turned on prior to applying cosmetic threads (shaded cosmetic threads option checked). Only way to get rid of the opaque surfaces is by removing the cosmetic thread from part. Unchecking "shaded cosmetic threads" option doesn't get rid of the opaque surfaces. Maybe this is how they envisioned it to work. For me, it is annoying to work in assemblies with opaque surfaces from cosmetic threads. Also have experienced some (not all) surfaces in transparent assembly parts will turn opaque. From the assembly must RMB on part, appearance, color and remove color.

Kman