In Qld you must have a
white light that illuminates all of your number plate, and that doesn't directly show light to the rear.
^^^ That would make a coloured number plate light illegal
But... if you wanted to colour your number plate light for show use only (wink wink nudge nudge know what I mean say no more) there are a few ways to do it.
You could paint the lens of the lights or the bulbs themselves.
You could buy some little bulb condom thingies. These are little coloured rubber things that slip over your bulbs and change the colour of the light.
Or if you're handy with a soldering iron you could get some high-brightness LEDs from Jaycar or Dick Smith Electronics and put them in place of your bulbs. Keep in mind you'll need current-limiting resistors for the LEDs. A quick search should turn up some good info on using LEDs in cars.