#5978A3

CSS

:root {
  --montreux-blue: #5978a3;
  --montreux-blue-rgb: 89 120 163;
  --montreux-blue-oklch: oklch(0.567 0.076 256.7);
}

.element {
  color: var(--montreux-blue);
  background-color: hsl(215 29% 49%);
  border-color: oklch(0.567 0.076 256.7);
}

Tailwind CSS v4

@theme {
  --color-montreux-blue: #5978a3;
  --color-montreux-blue-50: #e8f6ff;
  --color-montreux-blue-100: #d9e9ff;
  --color-montreux-blue-200: #c1d6f5;
  --color-montreux-blue-300: #a0b9df;
  --color-montreux-blue-400: #82a0cb;
  --color-montreux-blue-500: #6588b8;
  --color-montreux-blue-600: #56739c;
  --color-montreux-blue-700: #445c7d;
  --color-montreux-blue-800: #30445c;
  --color-montreux-blue-900: #212f41;
  --color-montreux-blue-950: #101b29;
}

SCSS

$montreux-blue: #5978a3;
$montreux-blue-rgb: 89, 120, 163;
$montreux-blue-light: mix(#fff, $montreux-blue, 20%);
$montreux-blue-dark: mix(#000, $montreux-blue, 20%);

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