Using face paint or masks as disguise was and still is part of many street festivals and performances. Superficial anonymity is part of the fun. The paint has to be dark enough and matt enough so that the way the light falls on the face is disrupted enough to conceal the features.

Woad isn't dark enough (and it is hard to get off). So maybe something closer to navy blue would be better

That said, I think there is case for limiting the situations in which concealing of the face in public is acceptable - I include large dark glasses as well as and paint, masks etc.

If people didn't keep going on about minstrels how many people in the UK, including non-white ones, would think of it? Most people under 50 will only have heard of the Black and White Minstrel Show as history. It's different in the USA obviously. How long do we have to keep going on about something that is gone for good.