I have been silent too. Sometimes it just happens so fast or you don't realise you should have said something till later. I think sometimes just understanding it's wrong is plenty ;) I remember when we were kids the police would search my aboriginal friend and not myself as I was white. I never spoke up and just stood while people walked passed judging my friend. It was upsetting for him as he felt humiliated, it was in his local suburb where people knew him. luckily my friend was able to move on and forget about it.