I think people should go lighter in the winter and fall and dark in the summer and spring. I'm saying that because in the spring and summer you wear brighter clothes and I think darker hair would go better with lighter and brighter clothes. Although, I'm sure anything would work.
Dark hair colors lighten up in the summer, it the suns way of saying "lighten your hair you twit." Okay, this would only be true if the sun was controlled by Cosmo or Style. But personally I think dark hari is good for summer because everyone is so blond, ya know.