The color of the gauge corresponds to the evaluation of the score in relation to the context of the trip. The evaluation depends on the traffic conditions of the route.
For example, a score of 8 out of 10 will be considered good in urban areas (green), and average for a trip on expressways (orange).
Indeed, when you drive on motorways, there are fewer environmental constraints (red light, priority, etc.). It is, therefore, easier to find oneself in conditions conducive to eco-driving (steady speed).