Lighter the colored the muscle the more activated it is
The four muscles that make up the quadriceps aremost activatedwhen the toes areneutral(VastusMedialis,RectusFemoris,VastusIntermedius,VastusLateralis)
Most people (I hope) no longer still do these variations on the leg extension, but this stands true for not only the leg extension. Your feet should always be parallel, like you are standing on skis, during your leg exercises. Even if your feet rotate during the exercise, shift them back to parallel ASAP.