In a small saucepan, boil the spinach in enough water to cover for 5 minutes. Drain, discarding the cooking liquid.
In a high-speed blender or food processor, blend the spinach and water together until completely smooth. If the mixture clumps or stubbornly refuses to blend, add more water as needed, 1 tablespoon at a time. Strain, if desired, and let cool.
Store in an airtight container for up to 2 weeks in the refrigerator. Add 1 teaspoon to icings, frostings or batter for starters to impart a green hue. Add more coloring, if necessary.