Prettier 2.0.5
Playground link

--parser css

Input:

foo {
  color: rgb(0, 128, 255);
  color: rgba(0, 128, 255, 0.5);
  color: hsl(198, 28%, 50%);
  color: hsla(198, 28%, 50%, 0.5);
  color: lab(56.29% 019.93 16.58 / 50%);
  color: lch(56.29% 19.86 236.62 / 50%);
  /* color(sRGB 0 0.50 1 / 50%); */
}

Output:

foo {
  color: rgb(0, 128, 255);
  color: rgba(0, 128, 255, 0.5);
  color: hsl(198, 28%, 50%);
  color: hsla(198, 28%, 50%, 0.5);
  color: lab(56.29% 019.93 16.58 / 50%);
  color: lch(56.29% 19.86 236.62 / 50%);
  /* color(sRGB 0 0.50 1 / 50%); */
}

Expected behavior:

foo {
  color: rgb(0 128 255);
  color: rgb(0 128 255 / 50%);
  color: hsl(198deg 28% 50%);
  color: hsl(198deg 28% 50% / 50%);
  color: lab(56.29% 019.93 16.58 / 50%);
  color: lch(56.29% 19.86 236.62deg / 50%);
  /* color(sRGB 0 0.50 1 / 50%); */
}

Notes:

• in general, use the modern comma-free syntax instead of the legacy comma-based syntax ("legacy" is how the CSS Color spec refers to it)
• use rgb() with the new syntax instead of rgba() (which is a legacy function)
• use hsl() with the new syntax instead of hsla() (which is a legacy function)
• consider being opinionated w.r.t. how to represent alpha values. IMHO using percentage notation is most readable, as it clarifies the range: e.g. 70% vs 0.7
• consider being opinionated about using the deg suffix where applicable, e.g. in the first parameter for hsl() and the last non-alpha parameter for lch(). Again, because this clarifies that the range lies between 0-360.
• the color() syntax is not yet supported by the parser, but that's a separate issue.
• optionally, consider being opinionated about preferring #rrggbba/#rgba hex notation over rgb() notation, or the other way around