#5976A0

CSS

:root {
  --montreux-blue: #5976a0;
  --montreux-blue-rgb: 89 118 160;
  --montreux-blue-oklch: oklch(0.561 0.074 257.6);
}

.element {
  color: var(--montreux-blue);
  background-color: hsl(215 29% 49%);
  border-color: oklch(0.561 0.074 257.6);
}

Tailwind CSS v4

@theme {
  --color-montreux-blue: #5976a0;
  --color-montreux-blue-50: #e9f6ff;
  --color-montreux-blue-100: #dae9ff;
  --color-montreux-blue-200: #c2d6f5;
  --color-montreux-blue-300: #a1b9de;
  --color-montreux-blue-400: #83a0ca;
  --color-montreux-blue-500: #6788b7;
  --color-montreux-blue-600: #57739b;
  --color-montreux-blue-700: #465c7c;
  --color-montreux-blue-800: #31435c;
  --color-montreux-blue-900: #212f40;
  --color-montreux-blue-950: #111b29;
}

SCSS

$montreux-blue: #5976a0;
$montreux-blue-rgb: 89, 118, 160;
$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);
}