Funny you should bring up these rare colors...
Today I was at my new go-to betta vendor, and walked out with 3 new boys. Why 3? Well, in the store, one of them appeared to be a solid colored dark plum-purple halfmoon. This place carries near- to show-quality bettas, so I didn't rule it out.
When I got home, it turns out he's actually a deep royal blue. Not really a hint of purple on him.
As a result, his name will be Prince, since Prince has the whole purple thing going on, and since he's actually royal blue.
I knew I was blue/green colorblind, but now it looks like purple might be amongst those as well.