In a large bowl, whisk the egg whites and vanilla until just blended. Gradually whisk in the sugar. The icing should be the texture of heavy cream. If it’s too loose, add a little more sugar; too dry, add a little water. Divide the icing among small bowls. Tint as desired. (I like to make red, green, brown, blue, and white.)
Transfer the icing to separate piping bags. Or use gallon-size freezer bags: Tuck one bottom corner into a pint glass, folding the top of the bag over the rim of the glass to make it easy to add the icing. Spoon in the icing, aiming for the corner of the bag and getting as little as possible on the sides. Lift the bag from the glass and twist it just above the top of the icing. Just before piping, snip a small piece from the corner of the bag. While piping, twist the top of the bag to push the icing down.