What kind of witnesses shall we be? It depends on who the “we” are.
I’m a person of color so I feel, hear, and experience racism and bigotry when it shows up. So I am and will be a witness who is loud in my protest of it. To many white people, my loud protest, by itself, is called “reverse racism.” So, to many I am a racist just because I protest. 

Whites can’t be witnesses against such things because they don’t know what racism and bigotry from other whites feels like. So even a little protest is seen as a monstrous act of reverse racism. They can empathize. They can march. They can voice vocal protests. And that’s all good. It needs to be done.

So even in the face of racist killings in our country and bigotry of the Trump kind, whites have to drum up the courage to make a stand against something they can't and don't and won't understand. People of color already have that kind of courage because it is a built-in mechanism for survival in America as we know it.