Place the ground beef in a large skillet over medium-high heat. Add the green pepper, red pepper, yellow onion and red onion to the skillet, and cook until meat is evenly browned and vegetables are tender. Drain excess grease from the pan.

Transfer the mixture to a slow cooker, and stir in tomato paste, water and red pepper flakes. Cover, and cook at the Low setting for at least 4 hours. The longer it simmers, the spicier it gets!