Start by sifting the confectioners' sugar into a large bowl to remove any lumps.
In a separate bowl, slowly whisk together the warm water, honey, vanilla extract, and a pinch of salt until well combined.
Gradually add the wet ingredients to the sifted sugar, stirring until smooth and fully incorporated.
Stir in black gel food coloring, adding a little at a time, until you achieve a rich greyish color. (See notes below).
Cover the icing with a damp cloth and let it sit for at least an hour to allow the color to set as well as allow any bubbles to rise.
Use a spatula to stir gently and remove any bubbles. Add more black food coloring if necessary, until you achieve a rich black color.
Lift a whisk or spatula from the icing to test the consistency. The icing should flow smoothly and settle back into itself within 10-15 seconds. If it's too thick, add a small amount of black food coloring or warm water, a few drops or a teaspoon at a time, until you reach the desired consistency. If the icing is too thin, thicken it by gradually whisking in more sifted confectioners' sugar, about a tablespoon at a time, until you achieve your desired consistency.
Notes
Start with a small amount and mix until you obtain a deep greyish tone. Cover the icing with a damp cloth and let it sit. Remember that the black color deepens as the icing sits. Mixing it a few hours in advance, is therefore ideal.