I always thought it was funny that Anarchists wave flags around but it's helpful in identifying them in the crowds.
You can use this for reference although there are many more flavours in the red/black communism theme. You will never see the Anarcho Capitalist flag in among this lot. They are arch enemies. Libertarians don't get along very well with communists but the rest of them are all far left of one flavour or another.
Groups flying these flags are referred to i the media as "counter protestors".