I paint my scouts in camo, without unit or chapter markings. There are a few reasons for this (both in/out of game):
1: It lets me use them with my Ultramarines or Blood Angels (when I still played
BA)
2: Their job is to gather information, not distribute it, so un-marked scouts don't give anyone who observes them any details
3: As initiates, they have not yet earned the rights to the chapter heraldry.
This worked fine until last month when I picked up Sgt. Talion. Who has multiple "U"s and company markings on him. I painted them black. The rest of the model is goblin/dark angels green base with bestial brown and chaos black camo (splotches on the hard armor/cloak, tiger stripes on the pants/sleeves)
I was on the fence weather or not do do the "tactical" black markings, or just the normal bright ones. After all, I paint an impression of camouflage, not actual effective camo, so there are plenty of other details that pop out and would earn you a sniper's bullet if worn in an actual war zone.
I think it turned out fine.
Edit: Picture