Plein air painting is all about light and atmosphere. Painting during sunrise or in the heat and intense light of the day can result in dramatically different works than if you are painting as the sun is setting. I've always preferred the latter. You get to sleep in but still get out there in time to witness the warm descent of the sun, which produces stunning colors and atmospheric effects in the sky and on the landscape.
To make the most of a sunset painting, use very little white and only when mixing a neutral or heightening a value. Another trick is to keep your canvas and your palette in the same light—so if the palette is in shadow, the canvas should be too. If one is in the light, pull the other into the same area so that you are looking at the same colors with the same light effects from palette to canvas and back.
When you are painting en plein air during sunset, let go of the need to visual define what you are looking at—a moored boat or a cabin on a hill—and concentrate on shape and color and reflected light. That, for me, is the most appealing part of outdoor painting. When plein air painting, I just want to respond to what I see in front of me—the rich reflections of sunlight on the water or how all the colors seem enriched when the sun emits that intense orange glow as it sets. It’s the best part of being out there. Enjoy!