Create a detailed illustration of mountains during sunset. The scene should depict a range of mountains with various peaks and valleys, capturing the tranquil atmosphere of the setting sun. Include elements like trees, rocks, and perhaps a winding river or a calm lake at the base of the mountains. The sky should show a gradient with clear outlines of clouds and the sun just above the horizon. The overall style should be realistic yet tranquil, focusing on the serene beauty of nature. The image should be suitable for coloring, with clear outlines and defined spaces.