Decorator Nate Berkus would love the way Earth looks in WALL-E; all browns and tans and neutral tones. WALL-E himself fits right in, with his black-and-yellow color scheme.
The Earth hasn't decided to color coordinate, though. It's dying. It's brown and grey because everything is dead and dirty… until WALL-E finds the plant. When WALL-E opens an old refrigerator, he discovers (next to the skeleton of Indiana Jones) a plant. It's the first instance of the color green in the whole movie, and the camera zooms in on it, slightly blurry, as if this plant is an alien that doesn't belong on Earth.
WALL-E, being the collector he is, scoops up the plant and puts it in a boot and takes it home. The rest you know: the plant is the driving force for the whole movie, and the object that will bring humanity back to Earth. This one little plant has a lot of power. When the Captain finally retrieves it, a leaf falls off, so the Captain waters it. "Just needed someone to look after you, that's all…" he says, a phrase that he believes could also pertain to Earth. That's why the Captain wants to return to Earth—to give it a little TLC. (He seems to forget that humans destroyed it. Earth would be just fine were it not for us ruining it.)
At the end of the movie, though, it seems that humanity is doing a good job rebuilding Earth. The plant has grown into a big tree—a heavy allusion to the Tree of Life. The last thing we see in the credits are the tree's roots growing from the boot underground. Everyone, even the plant, gets a happy ending.