I can't think of any colour that I've seen on Letizia that doesn't look good on her. (She makes me think of Queen Rania in this way.) Her complexion is just so that most colours flatter some of her features or accentuate something about her.
Black makes everyone look good, though it's not one of my favourite colours on Letizia. Nor is grey as it's a rather dull colour -- though it's dull on everybody, not just Letizia.
But I think very bright, vibrant colours such as reds and oranges (not everyone can look good in orange either) look great on Letizia. They make her whole face come alive.
And pastel colours such as pink soften Letizia's features, which are sharp and angular.