When margarine was invented, it cut into the butter market. The dairy lobby begged Congress for help and got it in the form of a federal tax on margarine of one quarter of a cent per pound on white margarine and ten cents per pound on yellow margarine. Obviously, since people were used to yellow butter, white margarine was unattractive and less competitive.
This discriminatory tax on margarine, especially yellow margarine, was challenged.
Would such a tax be improper? [McCray v. United States, 195 U.S. 27 (1904)]