What's the purpose for many mushrooms to have colorful caps? Is not it is in their interest to hide as well as possible? Or there are mushrooms that propagate by being eaten, like fruits?
Why not all the mushrooms are poisonous?
Mushroom is actually the reproductive organ of the actual fungal body beneath the surface. It have a high surface area to disperse or spread the spores.
If the eating predators are low or dispersing is not an option, then it will need other organisms to attract to that they can be spread, so they are not toxic. Analogous to fruits, they uses different colors which are non-toxic to attract.$^1$
Whereas brightly colored mushrooms are less toxic, whereas, most toxic ones are either brown or white.$^2$