red gold is simply gold which is alloyed with copper which gives it a reddish color. Often called : pink gold or rose gold.
depending on the amount of copper used will determine the saturation of “reD” or “pink” color.
You can find tons of vintage and antique jewellery with this specific combination of metals at auctions and jewellery stores alike. If you are specifically looking for antique items from royal families or collections i would recommend looking through a museums exhibitions. Perhaps the Met.
Hope this helps.