It’s for one specific strain of anarchism: Anarcho-communism (anarcho-syndicalism uses these colors, too, but it’s usually used for AnCom).

There are several designs for the different kinds of anarchism. Usually, it’s diagonal-black and some other color (e.g.: violet: anarcho-feminism, green: eco-anarchism, …).

Yellow doesn’t count, since ancaps aren’t anarchist.

Edit: typo