The four ninja turtles stand shoulder to shoulder on the sewer grate in front of a brick wall. The turtle on the left has a purple mask, the second from the left has a red mask, the third has a blue mask, and the one on the right has an orange mask. All four of the turtles have bright green skin and the wall behind them is decorated with a large green turtle shell. To make the ninja turtles pop, I would color them all in shades of green and make the masks in their respective colors, with the turtles wearing purple, red, blue and orange masks. I would also stay with the colors of green, purple, blue, and orange to further bring out the hero image of the turtles, shading their individual colors in different shades to add depth, and adding other small details like a few yellow stars that can be seen on the shell or in different areas of the sewer grate. Add in various shadows, highlights and additional colors to give the drawing more depth.