Over at Sluganimation.com there's a fantastic article by Justin Hilden that goes into the color design and theory of the Halloween classic and it's a pretty fascinating read.
Fences change from warm browns to cool blues. The green grass loses its yellow tinge and becomes more saturated. While we have been busy watching Linus wrestle his pumpkin homeward, the very air around the characters has changed and we can feel the coolness of night and a hint of the excitement that darkness will bring to Halloween.
Though it's a bit "looking behind the curtain" I think most of us are adult enough to be able to read the article and yet still enjoy the special in the years to come. It will probably make you appreciate It's the Great Pumpkin Charlie Brown even more than you may already do.
Check out the SlugAnimation.com article HERE.