What Causes Bright Green Stool?

my children have not been eating veggies, but took Tamiflu last week and now since they started the treatment have had bright green to dark green stools; not diarrhea, solid green poop of all shades. they have also been drinking blue juices for this same week. I have stopped both as of 3 days ago but yet they still have green poop. Should I be concerned??