This is my theory. As you might know, Shyamalan likes to use colors as a reference. Like in sixth sense and he used the color red a lot (watch the special features). I think his message has to do with the color yellow. The important scene is when Mark tells the little girl that when the ring turns yellow, it means your about to laugh. This really caught my attention. From that point on, I saw many yellow elements. The yellow elements were the things keeping them from getting infected. The main girl (i'm bad with character names) has a yellow bag. In the meadow where they stopped and the wind was blowing, the grass was yellow. The old lady's house had yellow wall paper and cabinets.

There were also a lot of little things. There were daisies in the meadow. When the showed these two women with gas masks, they had yellow candies on the table between them. The old lady at the house told Mark that she was drinking LEMON juice (it would have seemed random if you didn't thing yellow was the theme). Bees are yellow. I could be totally wrong about all of this.

Yellow means you're about to laugh and yellow keeps them from dying. I think Shyamalan is trying to say laughter keeps us alive. Which is why there are some goofy scenes in this movie. Its the laughter that keeps them alive.
Maybe its even about giving off good vibrations. THIS IS JUST A THEORY.

What do you think?