"It is a cold person."
Translation:C'est une personne froide.
Without context, "It is a cold person" appears to be incorrect English, but there are a couple of exceptional cases when it might be used. One is when we're referring not to an individual, but a hypothetical person, such as in northernguy's example above. Another case is when describing someone. For example, a stranger comes to the door without a jacket in winter, and Alice answers the door. Bill calls from another room, "Who's at the door?" Alice replies, "It is a cold person." (Though one would almost always use the contraction "it's" rather than "it is".)
Why is it wrong to say "C'est un person froid."? As far as I understand, it's just the masculine version of the same sentence right?
"une personne" has no masculine form. That noun is always feminine, whoever you refer to.
maybe this will help you:http://french.about.com/od/grammar/a/adjectives_4.htm
Isn't "cold person" more figurative than descriptive, or ar least can be, so this site would suggestions that either "froide personne" or "personne froide" could be acceptable - but is 'froid' used for this figurative meaning in French?