Learn Spanish (Latin America): Nuestro doctor es colombiano.

In Spanish, adjectives for nationalities and languages are not capitalized like they are in English. You will always write words like colombiano, mexicano, or inglés with a lowercase starting letter.

Why does the sentence use es instead of está?

The word es comes from the verb ser, which is used for traits and identities. Since someone's nationality is a core part of who they are, you always use ser to talk about it, rather than estar (which is used for temporary states or locations).

Why is it nuestro and not nuestra? Does it mean the people speaking are men?

No, it doesn't tell us the gender of the people speaking. In Spanish, the word for 'our' (nuestro) must match the gender and number of the person or thing being talked about. Since doctor is a masculine singular word, we use the masculine singular form nuestro.

How would this sentence change if the doctor was a woman?