It started out white and actually looked a lot like lard or shortening. Which makes sense, because they are all are just differently flavored forms of solid fat. But the yellow margarine was just considered more appetizing.

Also, there were a lot of unexpressed ideas packaged in with the color of the butter. There was "winter butter" which was very pale yellow, because the cows were fed hay and oats in the winter, and "summer butter" which was much deeper yellow because the cattle ate fresh grass and grains. Summer butter was considered much better tasting and more healthy. You'll see the same contrast today if you have access to "yard eggs" or eggs from free range chickens. The yolks are a deep orange instead of the pale lemon yellow of commercial eggs.

Then too, the recently discovered vitamins were a big deal. People were learning that yellow and orange foods were rich in vitamin A and hence healthier. Notice the second ad makes a point to say that their margarine has the same vitamin A content all year. This was in contrast to natural butter that had less vitamin A in winter.