https://lnikki.la/articles/sass-better-colour-based-on-brightness/

_luma.scss

/*
 * Calculates the sRGB luma of a colour.
 *
 * Math nicked from a great Thoughtbot article by Reda Lemeden:
 * http://robots.thoughtbot.com/closer-look-color-lightness
 */
 
@function luma($c) {
  $-local-red: red(rgba($c, 1.0));
  $-local-green: green(rgba($c, 1.0));
  $-local-blue: blue(rgba($c, 1.0));
 
  @return (0.2126 * $-local-red  +
           0.7152 * $-local-green +
           0.0722 * $-local-blue) / 255;
}

_pick-visible-color.scss

/*
 * Picks a colour from two options based on which one is more visible
 * on the given background colour.
 *
 * Usage: pick-visible-color($bg-color, $color-1, $color-2)
 */
 
@import 'luma';
 
@function pick-visible-color($bg, $c1, $c2) {
  $bg-luma: luma($bg);
  $c1-luma: luma($c1);
  $c2-luma: luma($c2);
 
  $c1-diff: abs($bg-luma - $c1-luma);
  $c2-diff: abs($bg-luma - $c2-luma);
 
  @if $c1-diff > $c2-diff {
    @return $c1;
  } @else {
    @return $c2;
  }
}

not-for-project-consumption.scss

$button-color: crimson;
 
button {
  background-color: $button-color;
 
  @if (lightness($button-color) < 50) {
    color: white;
  } @else {
    color: black;
  }
}

usage-example.scss

/////////////////////////////////////////////////////////////////
// variables/_colors.scss

$button-color: crimson;
// ...

/////////////////////////////////////////////////////////////////
// elements/_buttons.scss

button {
 background-color: $button-color;
 color: pick-visible-color($button-color, black, white);
 // ...
}

/////////////////////////////////////////////////////////////////
// main.scss (Sass's root file)

@import 'variables/colors';
@import 'functions/pick-visible-color';
@import 'elements/buttons';
// ...