Ask anyone who shot both stocks and they'll agree on one thing: Kodachrome 64 and Fuji Velvia 50 both produced slides loud enough to be instantly recognizable on a lightbox. Agreement stops there. The two films got to "vivid" through genuinely different chemistry, not a shared formula with different dials turned up.
Kodachrome's Dyes Weren't in the Film Yet
Kodachrome 64 is unusual among color films in a way that goes beyond its look: the unexposed film carried no dyes and no dye-forming couplers built into its layers at all. Almost every other color film — negative or slide, Velvia included — has its dye-forming chemistry built into the emulsion at the factory, ready to react during a standard development process. Kodachrome instead required its own dedicated process, K-14, which built the actual color dyes into the emulsion during development itself rather than the film carrying them from manufacture. The film you loaded into the camera was, chemically, still waiting on the part that would eventually determine its color — that only happened at the lab, during an unusually long and precise processing sequence.
That architecture is a large part of why K-14 processing was notoriously difficult to run correctly and why so few labs worldwide were ever equipped to do it — building the color into the film during processing, rather than simply developing dyes already sitting in the emulsion, left far less room for error, and a mistake at any stage had no equivalent failure mode in a standard process. It's also commonly cited as a contributor to Kodachrome's reputation for holding up unusually well in long-term storage, though the precise, verified mechanics of exactly how much this specific difference contributes to archival longevity, versus other factors in the film's construction, isn't something to state as a settled number here.
Velvia's Dyes Were Already There
Fuji Velvia 50 took the conventional architecture and pushed it hard in a specific direction. Its dye couplers are built into the emulsion at manufacture, the same basic arrangement as the vast majority of color film, and it develops through standard E-6 chemistry that any properly equipped slide-film lab can run. What makes Velvia loud isn't an unusual process — it's aggressive, channel-selective tuning of that ordinary architecture, deepening greens, richening reds, and intensifying blues well past what a more neutral slide film would produce from the same scene. Landscape photographers reached for it precisely for that reason: skies and foliage come back with a saturation punch that reads as more vivid than the eye remembered the scene being.
Same Reputation, Different Reason for It
Neither film earned its "vivid" reputation from the same lever. Kodachrome's saturation is inseparable from an entire dedicated development architecture that no longer exists in any commercially available process — reproducing that specific look today means working from scanned originals or simulation, not from a currently manufactured film, since K-14 processing ended with the last lab that ran it. Velvia's saturation lives in ordinary E-6 chemistry that's still processed routinely today, tuned toward an aggressive color response rather than requiring any unusual process step at all.
That's the honest version of "both are loud, but differently": one film's vividness was bound to a process step that doesn't survive it going out of production, and the other's is a tuning choice inside a process that's still very much alive. Knowing which kind of "loud" you're looking at changes what it would even mean to try to recreate either one.