With Tailwind 4 (4.0.9 currently); modifiers only seem to work with utilities that do have an argument, this works:

@utility text-* {
  --alpha: calc(--modifier(integer) * 1%);
  color: --alpha(--value(--*, [color]) / var(--alpha, 100%));
}

With text-default, text-default/50, text-blue, text-blue/50, etc.

This doesn't work:

@utility text {
  --alpha: calc(--modifier(integer) * 1%);
  color: --alpha(red / var(--alpha, 100%));
}

With just text, text/50, etc. as the modifier isn't parsed.

Demo: https://play.tailwindcss.com/ECphsE1vzA?file=css

Related discussion: #15775