Many poisonous mushrooms are extremely colorful. One hypothesis is that the colors serve as a warning to prevent animals from eating the mushrooms, much like the bright yellow and black stripes on wasps help to deter potential predators. Why do mushrooms need to deter predators when the majority of a mushroom consists of the underground mycelium?

Many poisonous mushrooms have bright colors that serve as the warning signal to animals that the mushroom is poisonous and by eating it they can die.  

Mushroom is a fungus and most of its part is consists of underground mycelium. The part of the mushroom which is above the ground is the fruiting body of mushroom and is umbrella-like. It is called cap in which gills underside of the cap. These gills contain spores that are disperse and produce more mushrooms.

Therefore as the cap is fruiting body and contains spore so to protect the spores mushroom need to deter predators.