Leviticus 14:37

And if, upon viewing the touch, he see that there is a touch in the walls of the house, and he seeth the cavities of a reddish or greenish colour, which appear to be deep in the wall;

And he shall look on the plague, and, behold, if the plague is in the walls of the house, he will see greenish or reddish cavities, and the appearance of them will be beneath the surface of the walls.