Change of colour, bike, car, bakkie. legal requirments?

Ok, I have asked a few friends and I get very conflicting answers.
So here goes, I want to respray my bike, car, Bakkie (Whatever), a different colour to what is on my registration document, licence description etc.
So what now?
I realise that if they (cops) stop me or check my licence or are following me and see the colour is different then I am in trouble>

So.... what do I do after I have resprayed my bike, car or bakkie?