Choose fresh grapefruit labeled pink or red, such as Texas red for these delightfully sweet-tart cookies.
In a large bowl beat butter with a mixer on medium 30 seconds. Add 1/2 cup of the powdered sugar. Beat until combined, scraping bowl as needed. Beat in water and vanilla until combined. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour. Stir in almonds and grapefruit zest. Cover and chill 30 to 60 minutes or until firm enough to shape.
Preheat oven to 325°F. Shape dough into 1-inch balls. Place 1 inch apart on ungreased cookie sheets. Bake about 15 minutes or until bottoms are light brown. Remove; cool on wire racks.
In a medium bowl stir together the remaining 2 cups powdered sugar and enough grapefruit juice to make drizzling consistency. If desired, tint lightly with food coloring. Dip balls in icing, allowing excess to drip back into bowl. Place on waxed paper. Top with coarse sugar before icing sets. Repeat with remaining balls. Let stand until set.