Does the question limit you to choosing between which and whose or could you use "I bought a pen, its color is blue".
It's a bit awkward but uses only one word as the space in your original question seems to request.
"Whose" is the possessive of "who" and technically refers to people not objects. (He-his, it-its, who-whose)
"Which" is being used as a pronoun so it needs a verb. (which is, which has etc.)