Dark Teal dye is a Winter Solstice dye, so it will only show up in Exploring Hoofbeat Plains when its winter time in the Northern Hemisphere. Otherwise someone could randomly sell one one day.

Ah, that is unfortunate. I was hoping it would be somewhat of a common thing to obtain for tack.

https://docs.google.com/spreadsheets/d/1CppSenbx1m5YbMtP9_C7WLo5-clEkO92_e3NudFh9ug/edit?pli=1&gid=0#gid=0

This at least shows some of the other seasonal dyes. I mean, take Spring for right now, I haven't sold them lately and I don't recall how many I had beforehand but currently? I'm up to 92 of the Dusty Green dyes and Thistle Dyes is brand spanking new for this year's Spring time. I almost have 50 of 'em.

Essentially whenever new seasons roll around you tend to stockpile them decently, its just a li'l downfall if you are a newer player that joins after a season ends.

They'll be back next Winter Solstice, right?

Yep, seasonal items come back each season

As Maki said, yes. Whenever it is the Winter Solstice in real life in the Northern Hemisphere, so December 21st, any Winter dyes, new and old, will return and remain in game to be found until the Spring Equinox comes around.

We are in Spring from March until June. Summer is June until September. Fall is September to December and Winter is December to March. Usually around the 20th/21st every 3 months.