Dak's helmet has always been blue.
His helmet is recycled from the vanishing "blue-squadron" in ANH.
Basically in the first film Luke was originally meant to be Blue 5 but the blue insignia and markings on the models interfered with FX shots so the squadron became red but the shots with blue insignia had already been filmed.
Gold Squadron was the original Red Squadron which may explain this toy:
Ady changed the Blue markings to red in ANH:R but there is still a continuity issue because in the battle only Luke has that insignia in red and all the helmets are distressed and weathered but as you can see these look rather shiny.