Whisk together powdered sugar, milk and vanilla extract in a medium mixing bowl until smooth.
3 cups powdered sugar, ½ tsp. vanilla extract, ¼ cup milk
Divide the sugar mixture evenly between 3 bowls. Add black gel coloring to the first bowl, whisking and adding more as needed, until you reach your desired shade.
Black gel coloring
Repeat with the orange and the purple gel coloring in the other two bowls. If you want even more colors, you can make a fourth bowl with green icing.
Orange gel coloring, Purple gel coloring
Line a cooling rack with a piece of parchment paper or foil underneath. Dip the top of a donut into the orange icing and let the excess drip off for a few seconds, then place it face up on the cooling rack.
12 donuts
Repeat for all the donuts, alternating icing colors.
Melt the melting wafers in a microwave safe bowl in 30 second increments, stirring well in between. Once melted, allow the candy a couple of minutes to cool.
10 ounces white candy melts
Place the candy in a piping bag and snip off the very tip. Use the candy to make spider webs, starting with the lines across and then connecting them with little waves.
Notes
You may want to practice making your webs by tracing the first few with a toothpick before adding the candy melts.