I just want to add that I live in the French Alps (3 hours away from the valley where Price studied his Swiss natives in the 1930's) and the cows have just this week been let out to pasture again after being inside barns for the winter. Whilst they are inside, their butter is pale yellow (because they eat hay that has been cut during the autumn) and soon the butter will be turning deeper yellow as they start eating fresh green grass again.
The movement of livestock is called 'transhumance' - where they are moved up to alpine quarters in the summer and are kept inside barns for the winter months.
I do not think it is the butter's color alone that should be an indication of its nutritional value, however; pastured butter is pastured butter, obviously better than intensively raised butter but the butter WAP was studying was alpine butter; made from milk that has been taken from cows who are pastured on the high alpine plateaux of Europe where the cows are completely free to eat medicinal flowers and all manner of other plants never found in the typical farmer's field (the cows get to choose what they eat - they are self-medicators).
Our butter tastes like no other butter I have ever tasted before and it's color/texture is never static, it turns from white to deep yellow throughout the year and the locals eat it when it is deep yellow (in the summer when the cows are high up on the mountains), it is also traditional in these parts to eat cheese when the butter is white - all the french mountain artisan cheese would have been made with summer milk and stored for consumption in the winter, insuring that the population receive the best nutrients all year round, the most expensive cheese here is still made with 'alpage milk'.
When it is in the fridge it is rock hard and when left on the counter, soft as margarine. I would be suspicious of butter that is soft even when in the fridge.
I am not sure about the butter in the U.S. but I would look for a farm outfit where the butter is produced 'by hand' so-to-speak and changes color throughout the year. If there are any 'artisan' dairies where you live, buy cheese in the winter and eat yellow butter in the summer (preferably from cows grazed on mountainous terrain where they have rough pasture containing a variety of different wild plants to eat).