Charles Schulz must have come up with Charlie Brown's "Crayola shirt," as I call it, as a way to make the character visually distinct from the others. It would not be overstating things to say that it is known the world over.
Snoopy edges still closer to humanity here. Notice that he has no thought balloons in these early strips. I might be wrong here, but I seem to remember that Peanuts was the first strip that used them to present a way for unspeaking animals to kinda-sorta talk.