#1570B3

CSS

:root {
  --french-blue: #1570b3;
  --french-blue-rgb: 21 112 179;
  --french-blue-oklch: oklch(0.53 0.132 247.2);
}

.element {
  color: var(--french-blue);
  background-color: hsl(205 79% 39%);
  border-color: oklch(0.53 0.132 247.2);
}

Tailwind CSS v4

@theme {
  --color-french-blue: #1570b3;
  --color-french-blue-50: #d9f9ff;
  --color-french-blue-100: #c7ecff;
  --color-french-blue-200: #a9d9ff;
  --color-french-blue-300: #81bdf8;
  --color-french-blue-400: #58a5e8;
  --color-french-blue-500: #2a8bd8;
  --color-french-blue-600: #2676b6;
  --color-french-blue-700: #1f5f92;
  --color-french-blue-800: #10466c;
  --color-french-blue-900: #0a304c;
  --color-french-blue-950: #001c33;
}

SCSS

$french-blue: #1570b3;
$french-blue-rgb: 21, 112, 179;
$french-blue-light: mix(#fff, $french-blue, 20%);
$french-blue-dark: mix(#000, $french-blue, 20%);

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