Twilight, that time when the sun has already set but there is still light in the sky, is one of my favorite times to shoot. The color of the sky makes a gorgeous backdrop to just about anything, especially when you use strong colors that stand out against the blue. I love shooting moving cars against a twilight sky; the red taillights provide a great contrast of colors. But sometimes it's difficult to get the right mix of trailing lights against the sky due to traffic flow. You can wait for a big flow of cars, or try a different technique--multiple exposures. As long as you have the camera locked down on a tripod and don't move it between shots, you can shoot in-camera multiple exposures or merge multiple shots together in PhotoShop. It's a fun technique to play with and can yield some beautiful results.
This shot of the Flatiron Building in New York is a 9 shot in-camera multiple exposure. The traffic was coming from both camera left and camera right, so the multiple exposures enabled me to record the trailing lights on both sides.