Have you set up a goal for that workout? The yellow ones are based on time, the blue ones are based on distance. Would suggest you have changed the goal for that type of workout. Or you have changed the way it is displayed.
Edit: to be clear, go to the watch and open the walk, in the top left there are three dots, select that and it shows you by color what the goal is, Green is an open goal, blue is distance, yellow is time and red is calories.