I grew out some of Ken's GDP in 2017 - they were feminized seeds I picked up from Attitude. I did two runs of it, the second run being clones of the first run. One pheno turned very purple (almost black) and the other stayed green throughout. I have pics in my journal if you want to browse what the end product looked like.
All in all I have no complains with the plants I grew. Solid aroma, great taste, great yield... but it's not something I'm itching to grow out again.
As others have said, Ken is kind of a douchebag and his breeding practices are questionable. Eric, who now runs DVG (Dungeon's Vault Genetics), shed some light to the situation a couple years ago here on RIU, you'd have to search for the thread. It's a good read if you can find it. I didn't learn about all of this until after I ordered the seeds, otherwise I would've purchased seeds elsewhere.
I've seen a few interviews with Ken where he's asked about the lineage of GDP, and he isn't very forthcoming about the details. If I recall correctly, he said the cut came from some indians or something to that extent. I don't believe the GDP gear he's selling nowadays is the same GDP of 5 years ago. Once again, if I recall correctly, the original GDP was lost several years back and the GDP being sold today is of totally different lineage.
If you are looking for buds that will turn purple then there's a ton of great genetics out there for you. You may want to look at DVG as an option, I know the OGKB that Eric uses brings out fire in a lot of his crosses. Oni is another breeder that you may want to look into, Tropicanna Cookies for example is a very purple leaning strain.