How do you translate 'yellow one' into Latin?

Yellow in Latin is "flavus" or "croseus".

One in Latin is "unus".

If you are referring to a person with yellow or blond hair you would probably use "flavus". If you are referring to a person's skin (cutis) you should use "croseus".

Latin usually places adjectives after nouns. So, if you are referring to someone or something as the "yellow one", you should say "unus croseus" or "one who is yellow."