Hmmm, I always throught it was blue. Not exactly like a bright blue, but somewhere along the of pointing towards a nice dark blue with towards a shade that maybe add purple… I think I am not making much sense, but hard to explain the actual colour
EDIT: Arcane actually varies in colours, just the same as Void and Life magics.
Arcane has Blue(Time Magic and half of normal Arcane Magic), Purple(Lunar Magic, other half of normal Arcane Magic) White(Replication Magic and weakened Time Magic), Pink(Time Magic), Green(Nullification Magic), Red(Blood Elf Magic) and Orange(Astral Magic and Tachyon Magic) coloration.