#3B728F

CSS

:root {
  --harbour-blue: #3b728f;
  --harbour-blue-rgb: 59 114 143;
  --harbour-blue-oklch: oklch(0.527 0.074 233.6);
}

.element {
  color: var(--harbour-blue);
  background-color: hsl(201 42% 40%);
  border-color: oklch(0.527 0.074 233.6);
}

Tailwind CSS v4

@theme {
  --color-harbour-blue: #3b728f;
  --color-harbour-blue-50: #e2f9ff;
  --color-harbour-blue-100: #d1ecfe;
  --color-harbour-blue-200: #b7daf0;
  --color-harbour-blue-300: #93bed8;
  --color-harbour-blue-400: #72a6c4;
  --color-harbour-blue-500: #518eb0;
  --color-harbour-blue-600: #457994;
  --color-harbour-blue-700: #376077;
  --color-harbour-blue-800: #264758;
  --color-harbour-blue-900: #19313d;
  --color-harbour-blue-950: #0a1d27;
}

SCSS

$harbour-blue: #3b728f;
$harbour-blue-rgb: 59, 114, 143;
$harbour-blue-light: mix(#fff, $harbour-blue, 20%);
$harbour-blue-dark: mix(#000, $harbour-blue, 20%);

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