Cate Blanchett's cornflower blue dress with fabric flower and flowing scarf was stunning enough by itself. The addition of button diamond earrings added a bit of sparkle, but didn't overwhelm.
The idea here is to make either the dress or the accessories the focus, not both.
For example, a black strapless gown with no adornment could carry a chunky diamond bracelet and necklace or a pair of chandelier earrings. Solid color dresses and neutrals are a great backdrop for colorful jewels.
On the other hand, the more ruffles, tiers and beading a garment has, the simpler the accessories should be. It doesn't mean you have to go bare (although if you are young and gorgeous, that's a big statement in itself), but you need to use restraint and choose smaller pieces.