I get all my useless knowledge from TV, only knew this because of last week's Mad Men.
When I was a kid back in the 40s, the dairy industry was not too happy with margarine. They had a law passed that required that the dye which makes margarine yellow could NOT be mixed before sale. The stick you bought looked like a block of lard with a tiny bead of orange dye in one corner. My job as a tot was to massage the container until the dye was evenly mixed in with the white block. Before mixing, the block looked quite unappealing... which is exactly what the dairy industry had in mind. As far as I know, nobody ever tried to pass margarine off as butter.
Also, back in the days of which @Ron_McNichol speaks, at least in Ohio where I lived, we called it Oleo, not margarine. But, I do remember having to add the real butter color ourselves.