There is a lot of space on this coloring page to be creative! The two gray tanks create an interesting contrast to the black pipes connecting them. I would start by coloring each tank a different shade of gray for variety. Start by filling the background with a light gray, then gradually darken the grays as you add more layers of color. To create further contrast, I would fill the black openings with a vibrant, dark color. Accentuate the black pipes by adding some different shades of gray and even a hint of black and white. Lastly, pay close attention to the details, such as the edges of the tanks and the curves of the pipes, by adding a hint of color or other decorations to make the page unique.