It's called Greenlight A Vet.  A symbol of appreciation and support for our veterans. Green is the color of hope, renewal and well-being.

Have you noticed all the green porch lights at night? No, it's not a Christmas decoration. It's called Greenlight A Vet supported by Walmart. We're encouraged to change one light to green in a visible location-on your porch, in your home, or at your office-and keep it glowing every day as a symbol of appreciation and support for our veterans. Then, share your support by taking a picture of your green light and posting it using the hashtag #greenlightavet.

Here's how it's described on the Greenlight A Vet website:

America’s veterans are some of our nation’s bravest, hardest-working men and women. However, it’s hard to show them the appreciation they deserve when, back home and out of uniform, they’re more camouflaged than ever. Greenlight A Vet is a campaign to establish visible national support for our veterans by changing one light to green.