Frank -- many authors actually advocate making the colour name invariable too in this case, so you'd write e.g. aux yeux bleu clair. In fact, I don't think I've seen a teaching grammar in the UK mention any other possibility. (Actual usage varies: some speakers apparently make the colour name invariable, but others make it agree.)
Edit: sorry, I think I misread your post because I saw you'd made the agreement in the first sentence.