Ways to RP a white mage:
1. You find a soulstone and practice. There were a TON of white mage zombies in Amdaphor HM surely they had soul stones. Maybe a soul stone was handed down in your family.
2. You learn it naturally from connecting with nature and soothing elementals. In HW your abilities grow and mature in ways even the Padjali did not know about because you traveled as that one padjali did ( I forgot his name sorry).
3. You buy a soulstone on the black market?
The thing is, we see scions using white mage spells, Y'shtola for example, she is capable of not only casting Holy but also Flow, both highly advanced spells. There's also Louisoix. My biggest suggestion with roleplaying a white mage, speaking as someone who has RP'd one since 2.0: Don't flaunt it, if your character is a white mage surely they know how taboo it is, they would only inform those closest to them. Plus honestly in RP I rarely found my character being a white mage was even that relevant. People just assumed they were a conjurer with advanced magick.
In summary: Roleplay what you want. Those who mind don't matter and those who matter won't mind.
Yes a cheesy line but appropriate.