Cream butter in large mixing bowl. Add sugar, eggs and vanilla; beat until light and fluffy. Combine flour, baking powder and salt. Blend into butter mixture alternately with sour cream. Stir in nuts. Cover and chill dough. Preheat oven to 350°. Drop dough by rounded teaspoonfuls onto buttered cookie sheets. Sprinkle with colored sugar. Bake 10 to 12 minutes. Remove from cookie sheets to wire racks to cool.