Translation:What is the color of your new door?
I think Fabien was asking: how do we know if "dveře" is to be translated as "door" or "doors in English? If the word is always plural in Czech, how can we count doors (one door, two doors... etc.)? Her house's door : dveře její domu Her house's doorS (as in "both the front door and the back door") : ?
In a general sentence like this, I (native AmE) don't think that "which" works as well as "what." "Which" is not entirely wrong, but it suggests that you are asking what particular color was selected from a range of known colors, and we lack that context. But if the Czech natives on the team feel that "which" is acceptable in the absence of context, I will be happy to add it.