Sift together flour, granulated sugar, baking powder and salt into bowl. Make well in center and add water, oil, egg yolks, grapefruit juice and half teaspoon lemon peel. Beat until very smooth. Beat egg whites with cream of tartar until stiff but not dry. Gradually pour egg yolk mixture over egg whites and fold in gently until just blended. Do not stir. Turn batter into ungreased 23 cm springform pan and bake at 180ºC 30 minutes, or until top springs back when touched lightly with finger. Invert onto rack and cool thoroughly. To make frosting, beat cheese until fluffy. Add lemon juice and 1 teaspoon peel. Gradually blend in powdered sugar and beat until well blended. Stir in food color. Crush enough grapefruit sections to measure 2 teaspoons and blend into frosting. Loosen edges of cake carefully and remove from pan. With serrated knife cut cake crosswise to make two layers. Fill with part of frosting and some of grapefruit sections. Spread top and sides of cake with frosting and decorate with remaining fruit sections.