Code:
local class = UnitClass("player")
playerVertexColor = {RAID_CLASS_COLORS[class].r, RAID_CLASS_COLORS[class].g, RAID_CLASS_COLORS[class].b, 1}
local db = Quartz:AcquireDBNamespace("Player")
db.profile.playercolor=playerVertexColor
db.profile.playeralpha=1
Quartz.ApplySettings()
Should do it.
edit:
on second thought, maybe not... After looking around in Quartz's code, it appears that there are no color options for the player cast bar at all. (ie. the Quartz_Latency addon has a color variable, but Quartz_Player does not).