The key to creating vivid iced sugar cookies is preparing two batches of icing for each color in the design. The icings will vary in consistency. A firm icing allows you to pipe a border that holds its shape. The stiff border fences in the second, thinner icing, which spreads across the surface of the cookie to create a smooth, even surface. Prepare a large workspace where you can spread all of the cookies out in a single layer overnight.
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Rolling and Cutting Cookies
Prepare a firm sugar cookie dough that's suitable for rolling and cutting. Roll cookie dough with a rolling pin on a sheet of parchment paper until the dough disc is about 1/8 inch thick. Bake the cookies according to your recipe, and then cool them completely. Icing won't harden if you pipe it onto warm cookies.
Every Color Is a "Section"
Think of each colored area on your cookie as a section. For example, a tree would have a green section and a brown section. Prepare and color two batches of icing for each color you need. Each section requires firm but smooth "border" icing and thin "flood" icing.
Border icing should have a consistency similar to firm toothpaste and hold the shape in which you pipe it. Flood icing should have a consistency similar to white glue; it doesn't hold a shape. Mix water or milk with powdered sugar to prepare simple icings that function like royal icings, if preferred. Color the icings as desired. You might prefer to border and flood the cookies with icings that are the same color, but you can prepare contrasting hues if you wish.
Filling Piping Bags
Fit a piping bag with a decorating tip that has a narrow, round opening roughly the same width as a strand of spaghetti. It will likely be labeled the "number 2" or "number 3" round tip. Fill the bag with border icing. Fit a second bag with a round tip that's the same size, and fill that bag with flood icing. Twist each piping bag above the icing and secure it with a rubber band to prevent the icing from spilling out. Keep royal icing or powdered sugar icing covered at all times -- in an airtight container or in a piping bag -- to prevent it from crusting.
Piping the Border
Hold the decorating tip about 1/2 inch above the top surface of a sugar cookie. Position the tip near the top edge of the cookie. Pipe a narrow border around one section of the cookie that you need to color. For example, trace the brown trunk section of a tree cookie with brown border icing. Allow the icing to set for about 10 minutes.
Flooding the Cookie
Hold the flood icing bag about 1/2 inch above the sugar cookie. Squeeze the bag gently to fill the colored section with icing, moving the tip of the bag as needed to fill the entire area. The flood icing should be approximately the same depth as the border icing; it should not spill over the border. Allow the icing to set for about one hour. The flood icing may look slightly uneven immediately after you pipe it. But the surface should become smoother as the icing settles. Pop tiny air bubbles in the flood icing with a toothpick as soon as you notice them.
Allow Cookies to Set Overnight
Border and flood the remaining sections of the cookies, pausing after each icing application to let it set, as needed. Add small icing details to the top of each cookie with firm border icing as needed. For example, pipe dots for the pupils in eyes after the white flood icing of the eyes has set for about an hour. Allow the icing to set completely overnight before packaging or serving the cookies. The icing will prevent the cookies from becoming stale. Store iced cookies in an airtight container after the icing hardens thoroughly.