Portugal and Spain had large Moorish populations (that were converted under duress to Catholicism). I would call them diverse - the Moors originated in North Africa. The Spanish Empire had a lot of interbreeding with the natives in Latin America. The Portuguese intermarried (and may marriage is a loose term here) with local women from Brazil to Goa. While the top positions were held by the "purebreds", an empire needs a large number of middle and lower middle class workers, soldiers, bureaucrats, etc. In Latin America, these obs were filled by the mixed races. The pure blooded natives were at the bottom, like the blacks in the USA.

As for China, as pointed out, their empires contained Chinese, Mongols, Koreans, Burmese, Tibetans, Naxi, Central Asians, etc. Those who don't know much about Asia might think all these countries are the same, but they are not. They speak different languages, have different religions and customs, etc.