#5679A3

CSS

:root {
  --montreux-blue: #5679a3;
  --montreux-blue-rgb: 86 121 163;
  --montreux-blue-oklch: oklch(0.567 0.077 253.5);
}

.element {
  color: var(--montreux-blue);
  background-color: hsl(213 31% 49%);
  border-color: oklch(0.567 0.077 253.5);
}

Tailwind CSS v4

@theme {
  --color-montreux-blue: #5679a3;
  --color-montreux-blue-50: #e7f7ff;
  --color-montreux-blue-100: #d7eaff;
  --color-montreux-blue-200: #bfd6f5;
  --color-montreux-blue-300: #9ebadf;
  --color-montreux-blue-400: #7fa1cb;
  --color-montreux-blue-500: #6289b8;
  --color-montreux-blue-600: #53749c;
  --color-montreux-blue-700: #425d7d;
  --color-montreux-blue-800: #2e445c;
  --color-montreux-blue-900: #1f2f41;
  --color-montreux-blue-950: #0f1c29;
}

SCSS

$montreux-blue: #5679a3;
$montreux-blue-rgb: 86, 121, 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);
}