I saw a Korean colleague with a debit card that also functions as a Climate Card?

What your colleague has is almost certainly linked to the Climate Card (기후동행카드), Seoul's unlimited public transport pass. The basic Climate Card lets you ride Seoul's subways, city buses, and the Ttareungi public bikes as much as you want for a flat monthly fee, currently around 62,000 to 65,000 won depending on whether you include the bike option, which is a great deal if you commute daily. Foreigners can absolutely use it.

The simplest route is the physical card: buy the plastic Climate Card for a few thousand won at a subway station convenience store or ticket machine, register it once on the official site or app, then charge it monthly and tap like a normal transit card. There is also a mobile version, though it currently works mainly on Android phones, not iPhones. The debit-card version your colleague showed you is a bank check card with the Climate Card function built in, offered by some Korean banks; to get that you generally need an ARC and a Korean bank account and you apply through that bank, but honestly the standalone physical Climate Card is the easiest option for most foreigners and does exactly the same thing. Just note the Climate Card is Seoul-focused, so if you travel far into Gyeonggi its coverage has limits; check the current zones on the official Seoul Climate Card site, which has English information.