How do you say brown duck in spanish?

There isn't a direct translation for "brown duck" in Spanish. You would need to describe it using two words:

* "Pato marrón" (pah-toh mah-rron) - This literally means "brown duck".

* "Pato de color marrón" (pah-toh deh koh-lor mah-rron) - This means "duck of brown color", which is a more descriptive way of saying it.

Both options are correct and will be understood by Spanish speakers.