Design is hard. A large part of this is choosing colors.
Use slightly blue-shifted gray instead of “perfect” gray.
Avoid black text on non-white background. That looks dirty. Instead use tinted dark color, or white letters on dark background.
Don’t use different hues of the same color. Colors with similar but different hues look off when combined.
Elements that are “above” (closer) should be lighter than those that are below (on the z-axis). Especially in dark mode.
Avoid simple linear gradients, esp. dual-color ones as background.
Use saturated colors sparingly. Use enough whitespace and neutral content color.
High contrast makes a design clean, as well as usable; Use dark enough text colors + whitespace; Use as few colors as possible — each additional color adds to the noise and decreases contrast.