I've grown out a lot of all purple plants over the years. I just harvested like 10 or so a few weeks ago. I don't find them to be less potent than the average plant. That being said I haven't found a true purple keeper yet, but I'm very picky about weed and have a high tolerance. Patients love it and get mad when I say I got rid of a purple mother. You got to remember not every one grows weed and has a super high tolerance.
That being said I only find a true keeper in about 1 out of 75 seeds or so. So it's really just a numbers thing the more purple plants you run the more likely you are to find one.
@bigworm6969 has the most consistently purple gear I have run. Almost all packs that claim to be purple have mostly purple phenos in my experience. If you can find Calypso that's been my favorite purple from him so far. Tropical Torte from him is very nice and potent but only turns slightly purple toward the end of flower, at least the 4 I had this run. Dirty flo from him had 3 out of 4 turn purple and have great smells but have yet to smoke much as most aren't finished being trimmed yet. I'm hoping I can find another pack of Calypso for the vault as I lost the best purple I've found in a clone mix up in my first run of them, I just ran the rest of the pack and am excited to try them and see how it compares.
I did just finish some Putang and the smell on all 5 is phenomenal with only 1 of them being green and 4 purple. Yield was average - high depending on pheno. Frost coverage is better on some than others. I haven't smoked em all yet as some haven't finished being trimmed. But overall I'm happy with the Putang. I hope to find a keeper because the smell is so good and people just love purple weed.