I was under the impression that Garp used Armament Hardening during Chinjao’s flashback…but I can’t check it right now.
I too think that Oda’s fire effects are really charmy and they just stand up whenever they are used (even in b&w). I like it best with this “painted” feeling, in opposition to Robby’s suggestion of adding glow and neon colors to it - it just feel more cohesive as a picture to me. The glow also reminds me of Marvel coloring, which most of the times I find really plastic and unattractive.
Oda’s Ice effects are cooler in b&w, though. Kazan seems to be riding a really pale wave instead of an ice avalanche lol