You’re not too bright, are you?

So is the person who doesn’t know

I was confused as well at first, but then I realised I need to think real literally: The one on the left is lit up and colourful, because he knows how to turn on a light; the one on the right looks as if he is in the dark (due to not knowing how to turn on the light)

Though you saying you’re “in the dark on this one” does make me wonder whether you did get the joke, and in fact I am missing a joke that you are making.