Hippos are big and fat. Have you ever seen a real hippo? Do you know what color does a real hippo look like? Hippos are mostly of dark grey color or of black color.
So first in this picture you color the hippo coloring it dark grey or black. But if you want to make the hippo a cartoon hippo you can color it pink or blue or whatever color you like. After coloring the hippo color the background, starting with the trees making them of green color.
There is also grass in the picture which should also be of green color. Than color the clouds with blue color and the background with a light color of your choice. Pick a nice shade of brown for the floor where the hippo is standing. Once you are done with this, this picture will look lovely.