Also, it is very important to give counter-examples: 'This crow over there belongs to the bird category. But the plane in the sky and the butterfly over there do not.' Or, more fitting the 'red' example: 'That stop sign and that traffic light are red. But this other traffic sign (can't think of an example) doesn't.'

And as well, this could be done with categories. 'Red is a color. Red is not a sound.'

I guess this one has something to do with confirmation bias, as cwillu suggested.