Yes, they did have lips. I don't dispute that, I'm not one of those weirdos who insist on drawing sauropods without lips or duckbills without cheeks. It's just that making them a different color at the front gives the impression of a beak like those of iguanodons. Not saying the color is wrong, just that the color scheme might confuse people who can't see the teeth so well.
BTW I don't think most sauropod lips were long enough to completely cover the teeth, especially not in big-toothed species. Bakker restored them that way but I take a more cautious route. The gums would easily have been covered though.