The mixture of coffee and milk would constitute a homogeneous mixture. This is because when the two substances blend together, the mixture itself takes one "same" (homogeneous) form. When the milk and the coffee mix, the resulting substance is a one-form color brown that takes the characteristics of the substance of which you have added the most. In other words, both substances blend in together to form a complete combination of the two.
Contrastingly, a heterogeneous mix is obvious to the human eye because its components often remain separate and you can see each one individually while they are still part of a mix. For example, oil and water would be heterogeneous because they are substances that, upon mixing, will immediately separate and remain individual substances. Another example of a heterogeneous mix would be cereal and milk. Unless the cereal is meant to be soluble as in some cases, what we often see in a bowl of cereal is a mix of a solid (cereal) and a liquid (milk) each adhering to their original forms without turning into one same thing. An even more simple example of heterogeneous mixtures would be a bowl of Skittles, where each small piece of candy has its own color and does not mix with the others.