On a different level,
TOS had bold use of color. Color TV was new, but whoever took the time to do set lighting had some mastery of the color wheel. What could easily become a mess is actually very refined, while extremely lively. Almost never monochromatic as that's stale and anything but lively unless saturation and brightness are maxed out, TOS frequently used schemes such as Analogous, Split-Complementary, Triadic, and even Tetradic. Or more. Even the season 2 cast shot from "I, Mudd" manages to balance a lot of "busy" while looking
iconic. To compare, TNG's palette is too often... beige and lead. Complementary is used, but generally with vibrant colors - note if the scene is of someone in a red shirt (e.g. Khan from Spacer Seed), the background is a suitable shade of green or teal that plays off that red and his skin hue and the result is sheer vibrance. Kirk wore yellow so expect a toned-down purple. In short, TOS is alive and exciting. TNG is... refined... or at least classy beige and lead... and for anything that's not a crew costume is eschewing the primary pigment colors (red, yellow, blue) for adornments but the secondaries (purple, orange, green - and generally as pastels but still unintentionally say "Happy Halloween" in the process). It still looks great, yet is just as bland in its own way. So much pastel you expect to see a marshmallow peep pop out.
TOS's uniforms were cheap and also made to exploit the shiny new color TV. The movies tried to do a more formal militaristic style with the maroon outfits, but then came TNG and eschewed that for more primary color hijinx - but with 80s angles, which work surprisingly well and hold up better than more generic fare that supplanted it by the time of their movies and spinoffs.
More on marshmallow pastel peeps:
If you like classical music:
For something more Monty Hill or Benny Python: