The colour of the potato depends on the type of potato, but yes, they are often brown! I suppose one answer to this question would be “Why not?”.
The potato that we eat is an underground storage organ called a tuber, and since it is under ground where it can’t be seen, the colour is not very important! It’s not green because green colours in plants are usually from photosynthetic pigments and that would be a waste of energy for the plant to make those pigments underground where there is no sunlight exposure.
Did you know that if you leave a potato in the light, it will eventually start making green pigments? You should not eat green potatoes because as well as photosynthetic pigments, the potato can also make other chemicals which mean it’s no longer good to eat.
There are also pink/purple potatoes, for example Branston potatoes, which get their colour from red purple and blue pigments called anthocyanins.
The potato skin can be brown because they have an accumulation of compounds called polyphenols and or alcaloids that adquire this color with the air contact, potatoes can be also pinkish because they accumulate red pigments (colours) and the skin adquire its colour. Also, if you take a potato from the soil, the skin will have the colour of the soil (dark brown) in most of the cases, after clean the skin will be light brown, pinkish or close to white.
It also depends on the potato variety as there are many different types from reds to brown to white. The skin of many potatoes in this country are also effected by fungal skin diseases making them darker brown (Scab), black (black dot or black scurf) or even slightly silver (Silver scurf). None of these diseases make the potatoes not safe to eat and you will find mild cases of many of them on sale in supermarkets. Was amazed at how clear and almost yellow potatoes in Australian shops look as they don’t have as much of these diseases. A farmers organisation has produced an interesting website with pictures of many of them: