Purpure is discouraged, but not prohibited, and the applicant expressed it as being their only preference.
On this I'd be a stickler for rules.
If it's a question of being discouraged, the prospective armiger was never discouraged on that score.
Secondly, he first said Blue and then choose Purple. So there's anyways an affinity to Blue.
On the choice of discouraged colours, I'd probably like to grant them only to such candidates who claims certain affinity based on education, regionalism or something similar and which cannot be replaced.
Many an armiger here might want to replace charges and or colour if them were permitted in the present case. Strict interpretation have been used in the past, and ideally should continue.