I see the possibility. Our modern green/yellow earth wires have a thin longitudinal green stripe along what is predominantly a yellow wire, so when viewed from a certain angle it can appear to be just a plain yellow wire. The proportions are supposed to be no less than 70/30, but I've seen some in which the green stripe is considerably less than 30% of the area.
On the radio color codes, red/yellow/black is also quite common on the radio end of sets here too for switched 12V, permanent 12V, and ground respectively, although sometimes it's red/orange/black, or some other combination. The actual loom wiring on the car can be almost anything, depending upon the manufacturer, although the speaker wires are often coded by left/right and front/rear, e.g.
Rear left: Green & green/white
Rear right: Green & green/red
Front left: Gray & gray/white
Front right: Gray & gray/red
or something along similar lines.
