#5A6FD4

CSS

:root {
  --royal-blue: #5a6fd4;
  --royal-blue-rgb: 90 111 212;
  --royal-blue-oklch: oklch(0.576 0.156 271.8);
}

.element {
  color: var(--royal-blue);
  background-color: hsl(230 59% 59%);
  border-color: oklch(0.576 0.156 271.8);
}

Tailwind CSS v4

@theme {
  --color-royal-blue: #5a6fd4;
  --color-royal-blue-50: #e7f3ff;
  --color-royal-blue-100: #d7e5ff;
  --color-royal-blue-200: #bfd0ff;
  --color-royal-blue-300: #9eb1ff;
  --color-royal-blue-400: #7f97fa;
  --color-royal-blue-500: #637bec;
  --color-royal-blue-600: #5468c7;
  --color-royal-blue-700: #4353a0;
  --color-royal-blue-800: #2e3d78;
  --color-royal-blue-900: #1f2a55;
  --color-royal-blue-950: #0f1739;
}

SCSS

$royal-blue: #5a6fd4;
$royal-blue-rgb: 90, 111, 212;
$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);
}