Grapefruit Cake: Preheat oven to 350 degrees. Sift together flour, sugar, baking powder and salt into mixing bowl. Make a well in the center of the dry ingredients. Add water, oil, egg yolks, grapefruit juice and lemon rind. Beat until smooth. Beat egg whites and cream of tartar separately until whites are stiff but not dry. Gradually fold the whites into the cake batter, folding gently with a rubber spatula until just blended. Do not stir the mixture. Pour into an ungreased 10-inch cake pan.
Bake at 350 degrees for 25 to 30 minutes, or until the cake springs back in the center when touched lightly. Place the pan on cake rack until cool. Run spatula around edge of cake. Carefully remove from pan. With a serrated knife, gently cut into two layers. After cutting the cake in two, slide a flat cookie sheet between the layers to lift the top one off; after frosting the bottom layer, simply slide the top back on.
Grapefruit Cream-Cheese Frosting: Let cream cheese soften at room temperature, then beat until fluffy. Add lemon juice and rind. Gradually blend in sugar and beat until well-blended. Add food coloring, if desired. Add reserved 2 tablespoons of grapefruit juice and blend. Divide the frosting evenly into two bowls. Roughly chop 2/3 of the grapefruit sections and add them to half the frosting. Spread this mixture on the bottom layer of the cake. Top with the second layer of cake, and frost the top and sides with the remaining frosting. Garnish each portion with a grapefruit slice. Serves six.