Just finished this painting, the principle for painting sunsets as with painting any piece of work that you want infused with a sense of light is the same, lighter and warmer towards the light source; darker and cooler away from the light source. The first time I really got the concept of warm-cool in the landscape context was when I saw this course notes on painting sunsets by landscape artist and teacher Richard Robinson. He explains very clearly the principles involved, and also how to take photographs so that we can recover the color information lost in the lights or the darks (depending on whether one focuses on the land or sky). Do check it out, its part of a larger mastering color course that can be purchased. His beautiful artwork is available at his website.
Watercolor Journal 