Thanks Mathijs!
It looks like custom painted prop hubs works as soon as the engines are shut down. But the spinning prop hubs seems to use the default textures (chrome). This also goes for the spinning props (propdisks).
I made custom files for the spinning props, but it seems like the default textures are used. Files that are affected (on my Widerøe LN-BEN): "DHC6_300_EX_WHELS_PAX_PROP_BLUR_ALBEDO.PNG.DDS" "DHC6_300_EX_WHELS_PAX_PROPHUB_BLUR_ALBEDO.PNG.DDS" "DHC6_300_EX_WHELS_PAX_PROPHUB_SLOW_ALBEDO.PNG.DDS"