The opaline mutation causes a larger area of white on the primary flights.
Opaline have a "V" area of lighter or no markings at all between the shoulders, as well as lighter head and neck barring. The amount of pigment from bird to bird can very greatly from bird to bird as you can see in the photos below.
Rather than only having a white or yellow edge to the wing feathers like a normal, opalines will have body color mixed in as well with the wing markings. Spangle like markings can be common on the upper shoulders, but this is not from the spangle mutation.
Many opalines will have a streak of body color down the middle of the main tail feathers.