I used the M16A2/A1 zero targets and I got yelled at by the Range Officers for using silhouette targets.
They only allow circle targets so I started making my own.
I found that a dark target makes it harder to verify POI using a scope at longer ranges because sometimes the holes that the bullets make appear dark and blend in. Of course sometimes the papers flare up at the impact hole so its easy to see.
What I did was make it gray for scope targets and black for iron sights. The gray targets are extremely easy to use with scopes because you can easily confirm points of impact.
The black targets obviously help with iron sights.
I'm too cheap to buy manufactured targets.
The old school shooters that I work with use paper plates and they use a sharpie marker to mark a 2" dot.