@vide Wisps, as a species, are a combination of a loose association of material and a dense concentration of magic. As such, they don't cast spells. Magic is a natural part of their physiology. As such, each race of wisp has its own natural magic type.
Dancing Lights use illusion because manipulating light and color with magic is as natural for them as lifting a finger. There is no casting process to speak of. If one tried to create a direct attack spell with light then it would require training much like learning how to use your hand in place of a knife.
Shifting Mists use conjuration because taking alternate forms and changing shape is more natural for them than those attributes are for the other two varieties. Conjuration should not be confused with summoning. The two are distinct and most conjuration is for direct attack or defense. I will clarify further below.
Glittering Clouds use Evocation because their nature is the most simple and volatile of the three varieties of wisp. Their magical nature is likewise clean-cut and direct.
------
Conjuration, for the purposes of this roleplay, can be likened to casting the Fireball spell. The caster conjures a volatile concentration of elemental fire which is then thrown or willed to strike a given target or explode at a given location. This spell does not work underwater due to the fact that the conjured elemental fire is drowned immediately upon materialization.
Evocation, for the purposes of this roleplay, can be likened to pyro-kinesis. It is the magical manipulation of existing natural energies. Magic energy is converted into thermal energy at a target point. Depending on the target material, this may produce a flame, steam, an explosion, or any other side-effect of a localized thermal spike. Notably, so long as the target is in sight, this version of evocation cannot be blocked.
I hope this clarifies what I have in mind. If not, or if you have any other questions, just ask. Sorry it took so long to reply but life has been... absurdly chaotic for me lately.