I would color the coloring page with warm and vibrant colors. The tree should be shaded with browns and greens, with vibrant reds and oranges for the firebird's feathers. The small girl should be colored with pinks and yellows for her dress and hair, and her doll should be light blues and purples. The dark forest should be colored dark blues and greens, and the home of Vasilisa's father should be dusted with golden yellows. Finally, the night sky should be a deep, star-studded blue. In the foreground, there should be accents of gold and silver, suggesting the gleam of stars and moonlight.