In QLD Yellow is a reserved colour for Emergency Vehicles only, along with Red, Blue, Green and Flashing White. So technically yeah is is a defectable thing up here, yet I have used them, funnily enough in the fog, and passed a copper and he didnt look twice.
They sell yellow spotties for cars at supercheap and I see heaps of 4x4 with them and red spotties here in rocky all the time, guess the police up here are pretty lax about it.