AMA sanctioned races followed the Pro AMA standard background color.
Black=125cc
White=250cc
Yellow=500cc
Ponca City, OK was an NMA track they assigned color lvl to experience lvl
Black=Novice
yellow=intermediate
Whie=Expert\pro
Lorretta Lynn's (AMA amateur championship)
had the same color scheme as Ponca
At the district lvl it was a fusterCluck as not many bothered to enforce the background color rules, hence the different color combos.