Give each color a token name, then choose the shared variable prefix:

use PhpColor\Color\Palette\ColorPalette;

$tokens = ColorPalette::parse([
    'brand' => '#7c3aed',
    'accent' => '#f59e0b',
    'surface' => '#f8fafc',
]);

echo $tokens->toCssVariables('theme');

The result is ready to place inside a selector:

  --theme-brand: #7c3aed;
  --theme-accent: #f59e0b;
  --theme-surface: #f8fafc;

toCssVariables() emits declarations only. It does not add :root, braces, or sanitize token names and prefixes.

See palette export or ColorPalette.

Export a palette as CSS variables