Here is the Gong link
I was looking for. It is one of my better posts.
You won't have any problems with 3/4" thick plate at those ranges with those calibers. It is just a little heavy to tote around and may not move well with long shots from the .22-250. If I was going to do it again I would try 1/2" and shoot the .300 win mag no closer than 400 yards.
White shows hits better than orange. I have never tried floro yellow though. It might work well.
If you want a more precise point to aim you could give the center of the target a quick spray dot of any color that you feel suitable like orange or black. I always just bring a can of white along to refresh the target mid session.