Red is a warning sign that something is off. Whether it's sickness, fatigue, stress, or something you're not entirely sure of, it means you need to back off duration AND intensity in your training. This is a day that you do not want to mess around with and push during; because it can set you back even further. On red days I always have my athletes run the lightest workout possible or not run at all. AND it's essential to cut the training time significantly so you can let everything calm down.