Some people love milk in their tea, and if you can't have or don't like dairy products, there are actually a lot of choices when it comes to adding 'milk' to tea. After trying many different types of milk with different teas, these are our findings: Black teas and Rooibos pair very well with almond milk. Black teas and Rooibos typically have a straight forward (simple) taste structure that can handle, and even sometimes need, the addition of outside flavors.
Green and white teas shouldn't have milk added, but if you must, use rice milk. Most green tea and white tea drinkers would never add milk of any kind to their tea as they want to experience the complex greens and subtle whites without dampening the taste. If you are adding milk to your green tea, only use a little bit of rice milk or you will complete with the tea's natural flavor. Rice milk is just milk enough for you to get away with adding it.
Oolong and Pu 'erh teas are best with soy milk. Soy is hearty and can pair well with the earthy flavors of Oolong and Pu 'erh.
Herbal teas and teas blended with spices, dried fruits and other flavorings, you should use rice milk or maybe soy milk depending on the strength of the tea, but you don't want to overpower the intended flavor that the tea craftsman is trying to help you experience, unless they didn't do a very good job!