Preheat oven to 325 degrees. Sift together dry ingredients then add egg yolks, oil, water, zest, and vanilla and mix well. When incorporated, fold in egg whites. Bake for an hour. When cake is cooled, slice in half and frost in between layers, on top and around sides.
Beat cream cheese with lemon juice until fluffy. Mash 6 grapefruit sections into small pieces. Add sugar to cheese mixture along with food coloring, fruit zest and blend will. Add remaining grapefruit.