The Yu-Gi-Oh! franchise has gone through a lot of changes; despite sharing its name with the card game that's the focus of most of its anime, Yu-Gi-Oh!'s anime career actually got its start another way -- with what fans have lovingly called "Season 0." Season 0 is different from Yu-Gi-Oh! Duel Monsters in may ways, as it adapts the darker, horror-themed first seven volumes of the manga, but perhaps the most obvious difference is the visuals.
Animated by Toei Animation rather than Studio Gallop (which worked on Duel Monsters all the way to Vrains), Season 0 looks almost nothing like the Yu-Gi-Oh! fans are used to. Part of this is due to its timing -- the show began airing in April of 1998, just shortly after volume seven of the manga was released. This means that the show had been in production for quite some time prior to airing, so when creating model sheets, the animators were referencing earlier volumes.
Unfortunately for them, mangaka Kazuki Takahashi had not yet settled into what fans now recognize as Yu-Gi-Oh!'s signature style; early volumes showcase much softer curves with almost a wave-like effect to them as opposed to the now-famous hair-so-sharp-you-might-cut-yourself, and it was this style that Toei was trying to match. This is further supported by the fact that Yu-Gi-Oh! The Movie, which was created after the series airing in 1999, much more closely matches the style that Studio Gallop ended up adapting for the screen.
But that doesn't account for the other changes that were made -- particularly in the palette department. For unspecified reasons, the character designers decided to switch up some of the colors for the characters, notably giving Seto Kaiba green hair. Subtler changes were made as well, such as coloring the girls' school uniforms orange instead of pink. Furthermore, the colors used for the characters are extremely saturated and bright, something that The Movie partially rectified, giving Kaiba back his brown hair, and blending the colors to appear less outlandish.
And the palette differences don't stop there either; fitting with the fact that Yu-Gi-Oh! was originally intended to be a horror manga with game elements, the backgrounds of Season 0 are notably darker than Studio Gallop's Season 1 -- and not just because they keep frequenting dark areas, like back alleys and lonely roads, either. The scenes use a notably muted palette with dull colors, which, when juxtaposed next to the characters, makes them appear even brighter.
Not helping is the high-contrast shading that Season 0 loves so much. Particularly noticeable on the boys' school uniforms, the dark shadows make the light areas appear even lighter, as well as sometimes containing a second shadow to further define the character's features. Studio Gallop, however, in suiting with the more lighthearted themes of Duel Monsters, used less dramatic shadows and subtler color choices that were more accurate to the manga.
Strangely enough, another reason Season 0 looks so weird is the animation itself -- Toei clearly put a lot of work into the series, making the characters expressive and fluid whenever possible. Studio Gallop on the other hand, due to characters being mostly static as they played a card game were free to put their resources instead into a consistent, well-polished style. As a result, Studio Gallop's animation is much stiffer, but no doubt they were able to save time and money by reusing animation between episodes. Toei's animation, however, is less on-model, but more expressive, and really sells the dark tone they were going for.