Combine buchimgaru, kimchi, kimchi juice, water, onion, egg, half of the red pepper, and half of the green pepper together in a bowl. Mix thoroughly until batter is pancake-like in consistency, adding more buchimgaru or water if needed.
Heat oil in a nonstick skillet over medium-high heat. Ladle batter to make 1 large pancake or several smaller ones; cook until slightly golden brown, about 5 minutes. Sprinkle the remaining red pepper and green pepper on top; flip and cook until set, 1 to 2 minutes more.