Rewatching chunks of Adventure and 02 now, it's so obvious to me that they made the backgrounds in such a way that the Digimon (and the main children) would easily pop against it. The colors are always balanced and chosen for contrast. Backgrounds are usually either subtle in color or very bright and airy, almost pastel, so the bold colors of the cast have more space to really grab your attention.
In 02 specifically, note how Archnemon and Mummymon's car is yellow, contrasting with their red and blue color schemes. In Adventure, most greenery tends towards a dark olive or a bright pastel, almost blue, which keeps Palmon separated from the bg.
Characters that are often seen together usually have matching color schemes: Takeru is yellow and aqua + Hikari is pink = CMY. Daisuke is tricolor -> Ken is grey. Mimi is pink, Palmon is green (and also pink!). Tai wears blue (and yellow), Agumon is yellow/orange. They look great against a blue sky, too.
I think this might be another reason why these two seasons in particular are so visually appealing to me. It's all about color, it's always on point. Color is so important, you guys. Color coding is also VERY present. I could go on and on
