Gas molecules are smaller than the wavelength of visible light. If light bumps into them, it acts differently. When light hits a gas molecule, some of it may get absorbed. After awhile, the molecule radiates (releases, or gives off) the light in a different direction. The color that is radiated is the same color that was absorbed. The different colors of light are affected differently. All of the colors can be absorbed. But the higher frequencies (blues) are absorbed more often than the lower frequencies (reds). This process is called Rayleigh scattering. (It is named after Lord John Rayleigh, an English physicist, who first described it in the 1870's.)
To make the pumpkin-and-gourd wreath, first cover a 20-inch foam form with sheet moss, using a glue gun to hold the moss in place. Loop a 3-inch-wide burlap ribbon over the wreath to create a hanger long enough so it can reach the top of the door and allow the wreath to hang at eye level. Insert wooden florist picks into the bottoms of pumpkins and gourds, and hot-glue the connection to secure. Once the glue has cooled, stick pumpkins and gourds into the wreath. Fill in with more moss, using a glue gun. To hang the wreath, secure the burlap ribbon with an upholstery tack hammered into the top edge of the door.