#5A75A3

CSS

:root {
  --montreux-blue: #5a75a3;
  --montreux-blue-rgb: 90 117 163;
  --montreux-blue-oklch: oklch(0.561 0.079 260.8);
}

.element {
  color: var(--montreux-blue);
  background-color: hsl(218 29% 50%);
  border-color: oklch(0.561 0.079 260.8);
}

Tailwind CSS v4

@theme {
  --color-montreux-blue: #5a75a3;
  --color-montreux-blue-50: #e9f6ff;
  --color-montreux-blue-100: #dae9ff;
  --color-montreux-blue-200: #c3d5f7;
  --color-montreux-blue-300: #a2b8e0;
  --color-montreux-blue-400: #849fcd;
  --color-montreux-blue-500: #6887bb;
  --color-montreux-blue-600: #58729d;
  --color-montreux-blue-700: #465b7e;
  --color-montreux-blue-800: #32435e;
  --color-montreux-blue-900: #222e42;
  --color-montreux-blue-950: #111b2a;
}

SCSS

$montreux-blue: #5a75a3;
$montreux-blue-rgb: 90, 117, 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);
}