While the milk in your refrigerator is a creamy white color, if you put it in the freezer, it will develop a yellow hue. This doesn't mean that it's going bad -- in fact, you can save frozen milk in the freezer for up to three months. It's just the components in the milk separating as they freeze, and when it thaws, it will return to its usual color.
Riboflavin's Coloring
One of the vitamins in milk is riboflavin, which is dissolved and scattered throughout the milk when its in a liquid state. The riboflavin itself is yellow, but because it is dissolved and mixed with other proteins and fats, its coloring can't be seen in this state. As the milk freezes, though, the riboflavin doesn't freeze as quickly as other ingredients, which makes it separate from them and become more concentrated. When it finally does freeze, its concentrated enough that its yellow coloring is clearly visible. After the milk thaws and the ingredients mix back together, the coloring will disappear once more.