As the snow melts, the soil warms.
As the soil warms, it becomes 40° Fahrenheit (and then warmer).
As the soil hits 40F, the bacteria that convert atmospheric nitrogen to plant soluble nitrogen wake up.
As those bacteria wake up, the soil is suddenly flush with new food for the plants.
As the plants eat, the plants start to green up again. One of the earliest is grass (right there in the surface of the soil…).
As the plants green up, they become more nutritious and more tasty.
As the grazing animals (like cows) eat, they aim for the tastiest plants.
As the cows eat the spring grass, they get flush full of fresh nutrients.
As the cows get a healthier diet, they have more ability to produce fatty milk.
As milk gets fattier, it turns a golden hue. (There’s some variance by breed. Guernseys have the best reputation.)
The golder the milk, the golder the butter.
In the winter it works in reverse. Of course, if the cow is not eating green grass, the color of the milk never changes. Since folks fresh off the farm in the 19th and 20th centuries knew that the brighter butter is better, the industrial scale butter makers (and cheese makers) started adding annatto, a seed, to change the color of butter and cheese. They got a little carried away and now we all think bright orange cheese is normal. All because of marketing departments wanting to imitate the color of +40° Fahrenheit.