Using a Y-shaped peeler, peel lemons, oranges, and grapefruit, taking off long, thick strips of zest.
In a medium saucepan, combine 4 cups water, 1/2 teaspoon salt, and citrus zest. Bring mixture to a boil over high heat. Reduce heat to medium, and simmer 10 minutes. Remove from heat and strain, reserving liquid.
Return reserved liquid, zest, and remaining 1/2 teaspoon salt to saucepan. Bring mixture to a boil over high heat. Reduce heat to medium, and simmer 10 minutes. Remove from heat and strain, discarding liquid and reserving zest.
In the same saucepan, add remaining 2 cups water and sugar. Cook over medium heat, stirring occasionally, until sugar dissolves. Reduce heat to low. Add citrus zest, and gently simmer about 45 minutes, being careful not to let the sugar caramelize. Remove from heat and drain.
Spray a piece of parchment paper with nonstick cooking spray. Place zest onto parchment, and let dry at least 6 hours or overnight. Store in an airtight container in refrigerator up to 1 week.