Well this is a new one. What does it mean? It was sent to me in an email and it’s interetsing because what the person seemed to mean by it is not exactly clear from the explanation Ciberdúvidas gives. First, here’s Ciberdúvidas
‘Numa distribuição circular das cores verificamos que o amarelo se opõe ao azul assim como o vermelho se opõe ao verde. Por esse facto, o contraste entre o azul e o amarelo bem como o contraste entre o vermelho e o verde são os mais bem definidos.
in Ciberdúvidas da Língua Portuguesa, https://ciberduvidas.iscte-iul.pt/consultorio/perguntas/ouro-sobre-azul/13660 [consultado em 29-11-2023]
Como o ouro brilha, o contraste é ainda mais impressionante. Esta combinação era muito frequente no vestuário das rainhas e até dos reis desde o Renascimento aos finais do século XVIII.’

So, basically, the first two sentences explain that Gold and Blue are opposite each other in a colour wheel so they are a strong contrast and they stand out well. So… it means it’s a contradiction then? the exact opposite? That didn’t seem to be what my email correspondent meant. But the second part of the text gives more of a clue: the fact that the yellow pops out so well makes it a popular combination in royal clothing (remember the Queen’s controversial hat a few years ago?). So that might mean it’s something different but in a good way.
And sure enough, if you look up the phrase on reverso-context, it gives most of the translations as something like “the icing on the cake”: something that would be a nice, striking addition to an already good thing, which I think is how it seemed to be being used. I think this is a rare example of Ciberdúvidas not being very helpful, but maybe that’s because they assume the person asking the question already knows the definition so it didn’t need spelling out. Fair enough!