It is illegal to manufacture or sell oleomargarine that is colored yellow to mimic real butter.

The Law Explained

In the height of the dairy wars, Wisconsin criminalized the addition of yellow dye to margarine, forcing it to be sold in an unappetizing pale white color. Citizens resorted to smuggling yellow margarine across state lines from Illinois in what were famously called 'oleo runs.' Though largely repealed in the 1960s, the legacy of the yellow oleo ban is legendary in dairy law.