White is a challenging color to change into other colors, as there is little contrast to begin with. If you simply add a layer of color, it will not look realistic.
The first issue is having a good photo of milk that does have tonality so that you can do something with it. Since I happened to be having my morning tea with milk, I took a photo of a glass of milk and made it chocolate. See the attached layered file.
It would take me hours to explain in detail how I did this, but you can look at the layers in the layers panel one by one from bottom up to see what types of things are needed. You can see the original selections in the channels panel.
One of the key factors is retaining the highlights from the original photo. This is where the quality of the original is of great importance: you cannot cleanly separate the highlights from a low-quality photo.