Zoro has purple flame , indicated with Sound effect in red :
yeah when Flame caught Ryuma's body
It's called
Boo : sound effect of flame .
Link :
https://www.baka-tsuki.org/project/index.php?title=Help:Japanese_SFX
http://thejadednetwork.com/sfx/index/ho
<> Purple flame is hotter than Blue flame by 24 times .
Zoro had blue flames since TS which can burn to crisp not like someone's blue flame that couldn't even burn clothes .
<> King had a Red Flame and with Haki it became Magma like flame :
<> So Adding Haki to Flames makes them stronger , and this is what happened when Zoro added a large quantity of Ryuo to his Blue Flame .