What do Prismana Orbs do?

The orb is the trigger, not the source. It only works on an Aniimo that is eligible, and only once that species' Prismana form has already been unlocked. So holding an orb does not mean you can make any creature in your collection radiant on the spot. The unlock comes first, and the orb is what you spend to actually perform the change.

What you end up with is a Prismana of the same species at Nova stage: an exceptionally rare, radiant variant with its own 3D model and a different color palette. How far the change goes beyond looks is not settled. It is described both as purely cosmetic and as a genuinely empowered form, and until that firms up you cannot assume an orb buys you a combat upgrade rather than a very good-looking one.

Getting a Prismana Aniimo without spending an orb

Nurturing also produces Prismana Aniimo. Whether that is the main route to them or a secondary one is unclear, which matters if you are deciding how carefully to ration orbs. If nurturing turns out to be the primary path, orbs are a shortcut on a species you have already unlocked. If it is the alternative, they are the reliable method and worth saving for an Aniimo you actually intend to use.

Not settled: Because it is unsettled whether a Prismana differs in more than appearance, there is no way yet to say whether an orb is a cosmetic spend or a power spend.