I fear that you are trying to know something akin to why a bard's empyreal arrow isblue-purple-ish rather than red or why a monk's elixir field is blue and not green or red or pink or whatever...
And I really doubt anyone even among the developers have had a truly thought out answer directly anchored into the lore either. The most probable answer is that of a standard colour association as white with pure, natural, healing magic, and purple as malevolent and/or destructive gloomy magic.
In terms of artistic believability, you could perfectly swap both and have conjury glow purple or even red, and thaumaturgy glow white, but especially in the case of the former, that would probably look very weird to most people, just due to those standard associations. Much like you associate red with danger and green with validation or safety.
Now then, one thing is sure, if the colour is tied to the element being cast, then since both conjury and thaumaturgy aren't limited to a few elements and actually have acces to the full aetherial elemental wheel, then it would probably mean that a conjurer's cane would glow purple when casting fire/ice/thunder and a thaumaturge's rod would glow white when casting earth, water or wind.
I somehow doubt that it's the case, and if there is any reason at all to begin with, I think that the colour is probably more tied to the style of magic and the tools they use to access it, which are diametrically opposed. So, white for a conjurer praying and pleading to elementals to allow them to access nature's bounty, no matter if they try casting Fire/Thunder/Ice ; and purple for a thaumaturge focusing and forcefully channeling their inner aetherial reserves through their weapon, no matter if they try casting Stone/Water/Wind.
And I really doubt anyone even among the developers have had a truly thought out answer directly anchored into the lore either. The most probable answer is that of a standard colour association as white with pure, natural, healing magic, and purple as malevolent and/or destructive gloomy magic.
In terms of artistic believability, you could perfectly swap both and have conjury glow purple or even red, and thaumaturgy glow white, but especially in the case of the former, that would probably look very weird to most people, just due to those standard associations. Much like you associate red with danger and green with validation or safety.
Now then, one thing is sure, if the colour is tied to the element being cast, then since both conjury and thaumaturgy aren't limited to a few elements and actually have acces to the full aetherial elemental wheel, then it would probably mean that a conjurer's cane would glow purple when casting fire/ice/thunder and a thaumaturge's rod would glow white when casting earth, water or wind.
I somehow doubt that it's the case, and if there is any reason at all to begin with, I think that the colour is probably more tied to the style of magic and the tools they use to access it, which are diametrically opposed. So, white for a conjurer praying and pleading to elementals to allow them to access nature's bounty, no matter if they try casting Fire/Thunder/Ice ; and purple for a thaumaturge focusing and forcefully channeling their inner aetherial reserves through their weapon, no matter if they try casting Stone/Water/Wind.
Balmung:Â Suen Shyu
