Some milk contains a lot of carotenoid pigments in the butterfat. When the butterfat is skimmed off and churned into butter, the butter is yellow.
Homemade butter you make by shaking a jar full of heavy whipping cream (or over-whipping your cream) is seldom as dark a yellow color as the butter you buy in the store. This is because commercial butter often has carotenoid pigments added to it, to make it look richer, or to at least match the buyer’s idea of what butter should look like.
You can extract your own carotenoid pigments to color your own homemade butter if you like.
First grate some carrots. Then melt some butter in a pan, and gently sauté the carrots in the butter. The melted butter will extract some of the carotenoid pigments from the carrots. The oily liquid that floats to the top is clarified butter, and is quite a bit darker than it used to be.
Cool the clarified butter. Now you can use a mixer to blend it into the homemade butter to get a deep yellow color.