For coloring this page, I would go for something bright and vibrant. The mandalas lend themselves particularly well to a rainbow theme, so I would use strong colors like blues and greens, purples, pinks and oranges to really make the designs stand out. You could experiment with shading different colors next to each other or adding in some metallic colors as highlights. You could also use some glitter glue or other embellishments for added texture and interest.i