In the stand mixer, cream the butter until it is lighter in color.
Add in one teaspoon of Violet gel food dye and mix until the color is fully incorporated. Use a rubber spatula to wipe down the sides of the bowl.
Add four cups of powdered sugar into the mixing bowl and mix until combined. Cover the mixing bowl with a towel or plastic wrap to avoid a messy kitchen.
Pour in one teaspoon of warm water and two teaspoons of vanilla extract, then mix until just combined.
Add in 1.5-2 teaspoons of black gel food dye. You will use 1.5 teaspoons to get a "passable" black and 2 teaspoons for a true black. Read my notes above for examples of when to use "passable" black.
The buttercream will resemble a dark purple if going for a "passable" black and a dark gray if going for a true black. Allow the buttercream to develop for several hours or overnight. This will result in a very, very dark purple or a truly black buttercream, depending on how much black gel dye you added.