On the prism knight, while it looks good so far, some bits of it feel somewhat incomplete. The meta illumination list for example feels it could use a little expanding. Off hand, it might be worth adding a note that a spirit shooter can treat their spirit shot as a blast illumination for the Prisms.

As a side note, themed costume does not actually specify that the permanent points need be spent on the element the prism knight is focused on.

A possible idea for a Prism would be one that allows a continuous damage on a successful hit. Possibly something like causing a fortitude or reflex save on hit, and if failed the target takes 1d4 damage of the chosen element per round for a number of rounds equal to the number of die of prismatic damage the prism knight possesses.

Possibly adding one or two illuminations specific to each element might also help. If you go with two, there could be one more flavourful utility type illumination, and one that is more combat effective. As an example of more flavourful illuminations, fire might provide an aura that gives off heat, making it so those in range need not make saves against cold temperatures, while cold might get an aura doing the reverse (both of which might specify an ability to be used outside of combat). That said, correspond illuminations to be granted by the other elements are harder to come up with. Acid just melts things, but based on that might get a corrosive touch that isn't much use in combat, but can be used for things like breaking locks, chains, etc. Electricity I'm unsure of in a fantasy setting, but in a modern one would be able to do something with electronics, otherwise best I can think of off hand is making light... Sonic, I similarly am unsure on, possible something communication related, such as an ability to converse long distance. They might even each get something later that if used for given Prisms, get added bonuses to make them combat capable (such as the heat/cold aura getting an extra d6 of damage when used with Elemental Assault). Combat specific illuminations could just be based off somewhat more iconic spells or abilities showcasing more unique aspects of each element.

Anyway, on the whole, I think a mostly matching chassis for each element is a good idea, but each should likely get a little something to show what is different about it compared to the others aside from what word you put next to the damage you're dealing/resistant to. If I come up with anything else, I'll let you know.

Owrtho