Cream butter or margarine. Add part of the sugar gradually, blending after each addition. Add remaining sugar, alternately with egg white, then with cream, beating after each addition until smooth. Stir in vanilla and salt. Divide into different bowls if you wish to make different colors, and tint delicately with coloring. Keep frosting covered to avoid crusting. If necessary, one or two drops of cream or milk may be added to maintain consistency