#647799

CSS

:root {
  --misty-harbour: #647799;
  --misty-harbour-rgb: 100 119 153;
  --misty-harbour-oklch: oklch(0.567 0.058 261.9);
}

.element {
  color: var(--misty-harbour);
  background-color: hsl(218 21% 50%);
  border-color: oklch(0.567 0.058 261.9);
}

Tailwind CSS v4

@theme {
  --color-misty-harbour: #647799;
  --color-misty-harbour-50: #edf5ff;
  --color-misty-harbour-100: #dee8fc;
  --color-misty-harbour-200: #c8d5ee;
  --color-misty-harbour-300: #a9b8d6;
  --color-misty-harbour-400: #8c9fc1;
  --color-misty-harbour-500: #7187ad;
  --color-misty-harbour-600: #607292;
  --color-misty-harbour-700: #4d5b75;
  --color-misty-harbour-800: #374357;
  --color-misty-harbour-900: #252e3c;
  --color-misty-harbour-950: #141b26;
}

SCSS

$misty-harbour: #647799;
$misty-harbour-rgb: 100, 119, 153;
$misty-harbour-light: mix(#fff, $misty-harbour, 20%);
$misty-harbour-dark: mix(#000, $misty-harbour, 20%);

.element {
  color: $misty-harbour;
  box-shadow: 0 0 0 2px rgba($misty-harbour, 0.35);
}