Grey for the balcony/advertising background? I was very skeptical about boarding over the Leitch balcony but the grey background brings out the adverts really well. Grey is a neutral colour so works well in bringing out other colours (in our case green!).
The barriers aren't a trip hazard because no-one will be walking through or over them on a matchday. The fans will enter from either side of the dugouts and the players/coaching staff will get to the dugouts from the pitch. They'll only be opened when vehicles/heavy machinery need access to the whole stand.
I hope that whole dugout area is astroturfed though, and as much of that area as possible to make it look as if the stand is as close as it can be to the pitch.