The Pokemon Torterra is a big, strong-looking creature with its large, sturdy shell on its back. The upper portion of its body is predominantly green, transitioning to brown towards the bottom. Its round head is bright green, with small black eyes and a large mouth. Its arms are short and stubby, while its legs are also fairly short and somewhat stubby. The shell on its back can vary from lighter to darker green, with some brown stripes and wavy-shaped marks.
For coloring this page, I would suggest a naturalistic approach that keeps the original colors true. The green should have some variation between lighter and darker shades, and the brown can be filled with earthy tones for a natural look. A bit of white and light grey can be used to help create a sense of depth and give the shell on its back more definition. The eyes, mouth and stripes can be colored in with a darker shade to contrast the lighter colors of the body. I believe that this approach would bring out the natural beauty of Torterra.