Sift together flour, sugar, baking powder, and salt into bowl. Make well in center and add water, oil, egg yolks, grapefruit juice and 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 9 inch spring form pan and bake at 350 degrees 30 minutes, or until top springs back when touched lightly with finger.

Invert onto cake rack and cool thoroughly. Loosen edges of cake carefully and remove from pan. With serrated knife, cut cake crosswise to make two layers. Reserve a few fruit sections for frosting. Fill with part of the Cream Cheese Frosting and grapefruit sections. Spread top and sides of cake with frosting and decorate with additional fruit sections.

Cream Cheese Frosting: Cream butter, then add cream cheese. Beat softened cream cheese mixture until fluffy. Add lemon juice and peel. Gradually blend in sugar and beat until well blended. Stir in food color. Crush enough grapefruit sections to measure 2 teaspoons and blend into frosting.

After frosting the cake, sprinkle it with the sugars, then place grapefruit sections on the top of the cake.