This rat looks so delightful and mischievous! The coat is a lovely shade of brown, so you could accentuate this by coloring the rat in all sorts of brown tones. For example, you could use a light brown for the belly area and a darker brown for the back and face. To bring out his bright beady eyes, a dark brown should be used to color the pupils, with a lighter brown for the eye-stalks and lids. His teeth could be a brilliant white, while the open mouth could be painted in pink. His long tail could be filled in with a chocolate brown color. The background could be painted in a range of greens and browns, using two or three shades to create a harmonious scene. Finally, you could outline the rat with a black pen to give him a bit of definition.