Why is it called the Purple Sapphire, when it is yellow?

I have taken many pictures of this lovely little butterfly....which seems to be yellow and red and brown.....

Well, last week I finally came across this lovely one., with its wings out in the sun, and all was clear...


Note the use of fill flash in the image below, which I think crispens up the image and allows me to shoot at 160iso and not 2,000


This species of butterfly exhibits slight sexual dimorphism where the males have a purplish blue patch on the dorsal side of the forewing while the females do not, as shown below.....



FYI...this is one of 300 bugs in my book THE BUGS OF HONG KONG....if you are already thinking of a Christmas present, then this could be it! Out of my 2,000 print run I think there are only about 50 left for sale....so if you did want one, I would urge you to order one now....and 50% of the proceeds go towards a local NGO....as if you needed another reason.

https://www.wildcreatureshongkong.org/orderbooklet