Use equal parts red, blue, and green food coloring
Add a small amount of each color to a bowl
Mix thoroughly until the color looks very dark
Add tiny amounts of black food coloring if available to deepen the shade
Adjust with more blue or green if the mixture looks too red
Add a drop of yellow only if the mix looks too purple
Stir until the color is as close to black as possible
Test the color in a small amount of frosting, batter, or liquid before using more
Let it sit for a few minutes, then check the final color again
Add more coloring gradually until the desired black shade is reached