Dr. Seuss had a complicated relationship with color. Horton Hatches and Egg is actually the first Seuss book in color since his very first book, To Think That I Saw It on Mulberry Street (source). We don't know why he decided to go all crazy and add mint (pistachio?) green and crimson to the black and white, but we know we like it.
Between the different colors, the humans and creatures, and the loopiness and sharpness, the illustrations in Horton are filled with contrast—just like the story. (Funny how that works.) The drawings are done in Seuss's inimitable, instantly recognizable style, but the delicate balance between light and heavy sets Horton apart.
Plus, this is about as far from the bright, primary colors of The Cat in the Hat land as we can get.