A normal pattern is always black and the black coloration is always on the inside of the feather.
A bird with an opaline mutation has a higher white ratio than black ratio on the feather of the neck. The bars look grey instead of black. Their body color usually runs through their wings as well. And if you look at their tail feathers the shaft of the feather is usually black and the feather is light.
opaline budgies have reduced black markings,almost absent on their heads,forming a V shape on their back(shown in red in the picture).Also the black markings are bordered with body colour instead of base colour:green instead of yellow for green budgies and blue instead of white for blue budgies.