Witty humour

Green is usually interpreted as a "go" signal, so whilst the sign says "stop", it implies that the poster is using an ironic tone to indicate and express the text above the picture.

It's green?

Tt is a hue of color visible to our range of sight. "Green means Go" is a phrase taught to children about road safety.

Apparently not to my range of sight it's grey like every other stop sign I see :/