Well there's not that many really, and a good portion of them have "purple" in the name which helps identify them lol. Ofc by default being purple doesn't really mean anything really, I believe with some genetics you can even turn plants purple if you shock them with cold... however if its just something you want to find;
- There's a purple pheno of Shoreline which is probably my favourite purple strain and isn't too hard to find on some menus. Not all shoreline you'll find will be purple though, for example Mr K's (edit - no, memory fail lol, it was Het Ballonetje) had a batch a couple of weeks ago that was decent but not purple at all.
- Iirc I've had Grandaddy Purple (or maybe Purple Urkle) from Grey Area before, but haven't seen a recent menu. Maybe seen one of them at CS 137 too.
- I've seen Grape Ape around, Dampkring maybe, but not for a while. That tends to be purple I think.
- Voyagers had a Purple Haze on their menu when I was in, didn't look at it though, seemed very pricey if it was traditional Purple Haze but I guess maybe it wasn't idk.
- Oh, and the Blueberry Muffin strain my mate got from CS Johnny was kinda purple, took a (slightly poor) pic. I thought it was a nice enough smoke for what I tried of it;
Hope that helps, menus change all the time though so you'll probably just need to check about and hope you hit lucky.