Colombian is how we turn the country name Colombia into “of Colombia” or “a person from Colombia” — the South American country with coasts on both the Pacific and the Caribbean, and with Bogotá as its capital. As an adjective: Colombian coffee, Colombian music, a Colombian city. As a noun: a Colombian, the Colombians. It follows the same quiet pattern as Canadian or Brazilian: place + -an. It is the normal, polite word — not slang, not a joke, not a nickname. Keep the capital C. Though actually, you will also see Columbian with a u. That spelling usually belongs to Columbia (the US or Canadian name), not to the South American country. If you mean the country, Colombian with an o is the one you want. People stretch Colombian in ordinary ways any nationality word stretches: a Colombian passport, Colombian food, a Colombian accent. None of that changes the core: it still points to Colombia. It is not the country name itself. The country is Colombia; a person from there is Colombian.