"The spell is called "Prismatic Spray", not "Taste The Rainbow"." (3.5 Domain, PEACH)

[Noctis Vigil]

They do. Prismatic Grasp does its effect once, Prismatic Missile does it once for every missile. Prismatic Mist is an area effect that does its damage to every creature inside it every round for 1 minute per level, which means if you trap a foe in it it will be taking a lot of nasty effects over time. Overall, Prismatic Mist is far more powerful; you just need to keep things in it for it to work.