O beautiful for spacious skies
For amber waves of grain
For purple mountain majesties
Above the fruited plain!
America! America!
God shed his grace on thee
This line refers to the purplish glow that mountains have from afar. This is caused by  atmospheric perspective . When there is dust in the air, red light (which has longer wavelengths) becomes more scattered. This visually mixes in with the blue from the sky, and creates an illusion that the mountain has a purplish tint to it 

America the Beautiful