ToxicRain is a small mod that makes rain poison players. It can also be configured to do so for snow, as well as water blocks. It provides a potion that protects from the rain (but not other forms of poison), which can be made by combining a potion of poison with sugar in the brewing stand.
Rain and snow are colored brown-greenish when they are toxic. The color is configurable and the system can be disabled entirely.
Players can be given a delay/grace periods, during which they will not be poisoned by rain. A delay can be applied through the /toxicrain command, the CraftTweaker integration, or a variety of config options. By default, players receive a 30-second grace period upon their first spawn and a 10-second grace period upon respawn.
The mod was inspired by spacechase0's old Toxic Rain mod, which is no longer available.