How does Ray Bradbury use motif when he says "the five spots of paint--the man, the woman, the children, the ball--remained. The rest was a thin charcoaled layer."
I'm not entirely certain that this can be used as an example of a literary motif; many definitions of motif require that the symbolism be repetitive, and this is the only instance of humans being reduced to ash or shown as silhouettes in the course of this story, or the whole of The Martian Chronicles.
If this is a motif, the symbol involved may be the fleeting, thin and superficial imprint of human life on the world; other aspects of the story are dedicated to depicting how any lingering effects of humanity's absence are slowly being swept away by nature.
It might also be considered motif because of the way Bradbury tends to call attention to details by simply reiterating them; it may be unnecessary to say that the five spots are "the man, the woman, the children, the ball", but this may serve to remind us that these were once living things, not just shapes on a wall, and that they therefore need to be seen as the outcome of horrific human actions rather than just curiosities.