Alien costumes will commonly be green and silver, because the stereotypical alien is green, and it wears a silver or green outfit. In order to make your own, you'll want a headband, tinfoil, two small springs from retractable pens, glue, clear packing tape, a green long-sleeved shirt or sweatshirt (and sweat pants: green or black), and (for the make-up) baby powder, cornstarch, shortening and food colouring. This doesn't take long to make (no more than a couple hours, if you already have the materials) and it's fun for you and your kids.
Glue the springs from the retractable pens to the top of the headband (one on either side), and wait for the glue to dry. Reinforce the glue with tape. Cover the headband and springs with tinfoil (taping it where necessary to keep it on). Crumple up another piece of tinfoil into two small balls, and attach them to the top of the springs with packing tape. This is the alien's antennae.
Dress the alien in the chosen clothes before beginning the make-up. Mix one part cornstarch with one part shortening, and add green food colour. Cover the alien's face with this make-up (using hands and fingers). With a cotton ball or make-up brush, apply a very thin layer of baby powder over the make-up.
Once you've put on the green make-up, the antennae and the green clothes, try adding some finishing touches with what you've got around the house.
For example, cover the alien's boots or shoes with tinfoil. If there are toy glasses around, wrap tinfoil around them for the alien to wear. Or, if the alien will be trick-or-treating in this costume, take a small bucket and cover that with tinfoil, to hold candy in.
If you'd really like to go the extra mile, make a jet pack. Find a cardboard box that would cover the alien's back. One one side of the box, tape two cords or ropes like straps of a backpack, to hold the box on the alien's back. Cover the box with tinfoil. On white scraps of paper, draw dials and buttons with a magic marker. Cut out the dials and buttons, then glue them to the back of the "jet pack."