#5B6CD1

CSS

:root {
  --royal-blue: #5b6cd1;
  --royal-blue-rgb: 91 108 209;
  --royal-blue-oklch: oklch(0.569 0.155 273.3);
}

.element {
  color: var(--royal-blue);
  background-color: hsl(231 56% 59%);
  border-color: oklch(0.569 0.155 273.3);
}

Tailwind CSS v4

@theme {
  --color-royal-blue: #5b6cd1;
  --color-royal-blue-50: #e8f2ff;
  --color-royal-blue-100: #d8e5ff;
  --color-royal-blue-200: #c1cfff;
  --color-royal-blue-300: #a1b0ff;
  --color-royal-blue-400: #8196f9;
  --color-royal-blue-500: #677aeb;
  --color-royal-blue-600: #5767c6;
  --color-royal-blue-700: #4553a0;
  --color-royal-blue-800: #2f3d77;
  --color-royal-blue-900: #202a54;
  --color-royal-blue-950: #101739;
}

SCSS

$royal-blue: #5b6cd1;
$royal-blue-rgb: 91, 108, 209;
$royal-blue-light: mix(#fff, $royal-blue, 20%);
$royal-blue-dark: mix(#000, $royal-blue, 20%);

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