Add the red, green and blue food coloring. Shake or stir to combine.
Add more food coloring if needed.
Store in an airtight, food-safe container.
Notes
Rum, gin, tequila, bourbon and flavored spirits can be substituted for the vodka. You can also use black food coloring instead of the 3 colors if desired. If making a larger quantity, use 3 drops of each color for every 8 ounces of alcohol for the strongest color.