It's a very different style than Calvin and Hobbes, but my favorite comic in color is "The Wrong Place" by Brecht Evens. The paintings in this book are so beautiful and full of details and I love his technique of using gouache and watercolors. Other comic books / graphic novels in color that I like are: "Casa Transparente" by María Luque, "Patience" by Daniel Clowes, "Turing" by Robert Deutsch, "Rosalie Blum" by Camille Jourdy, also Tara Booth.