Euglena is green because it contains chloroplasts. The structure located at the base of the tail (the flagellum) is called the photoreceptor. The photoreceptor detects light, while the chloroplasts produces sugars through photosynthesis.
The Euglena was green because it had chloroplasts in it, just like plants. The body structure at the base of the tail that relates to this is the photo receptor. These two enabled it to photosynthesize food like plants, but it could also take food from its surroundings like animals.
