I must admit that I was about to choose purple, but it seemed too typical, so after being about 25 minutes thinking, I ended up choosing white because it seemed the most curious.
Lore wise a purple saber is actually perfect for Cal! Usually those that wield a purple saber have an understanding of both the light and the dark side. Mace Windu wielded and famously battled with his darkside even creating a new form of combat called Vaapad which taught the user to use the darkside of the force while fighting. I.E using ones emotions.
With what Cal goes through in Survivor (No spoilers) purple makes perfect sense!