I bought a pen, ____ color is blue.

Here, should I use which or whose? I'm thinking of "whose".

I would rewrite it: "I bought a blue pen" or "I bought a pen which is blue". "I bought a pen, the color of which is blue" sounds better, but still stilted and non-native, than "I bought a pen whose color is blue" (makes the pen seem more animate, perhaps in a Harry Potter novel).

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.)

Is the pen itself blue or is the ink blue? If the teacher says you'll need a blue pen, you'll need a pen with blue ink. If I bought a blue pen to match my suit, the body of the pen is blue.

I would recommend matching the ink, rather than the body of the pen, to your suit . That way when the pen leaks no-one will notice.

>I bought a pen, ____ color is blue.

I bought a pen, its color is blue
and boy am I glad, the ink is blue too.

(easy to amuse)

Roses are red, violets are blue
hey whaddya know... pens are too.

(I don't think we should give up our day jobs in favour of becoming poets Branshea!)

Didja hear what happened to the Blue pen?

...It blue up! hahaha...oh! Never mind. Suspire

I'm so happy I red that.

no, don't reply! it's a trap!

"I bought a pen, its color is blue"

Some grammar mavens would say that this sentence contains an error which is called a comma splice (link). You might rewrite it one of three ways:

1. I bought a pen. Its color is blue.
2. I bought a pen; its color is blue.
3. I bought a pen, and its color is blue.

JFTR, comma splices don't bother me, but they do bother English and ESL/EFL teachers I have known. I don't usually correct other people's typos or mistakes, but, in this case, it's for callithump's benefit.

Too late! This thread is already becoming coloured by polychromatic punishment!

Orange you glad there is a limited number of colors?

You may be green in the unlimited world of colors.

> comma splice
zmjehzd, that really sounds like something awful and unforgivable. I hope I'll never again write or copy such thing ! Comma splice! Ouch!

perhaps you are relating it to the 'colon splice', which is sometimes a follow-up to the ever popular colonoscopy.

-ron o.

OWWWWWW!!!

I'm all thumbs when it comes to splicing. I let my husband hande that sort of thing. Much safer.

I was thinking it sounds like something I learnt to do in the Scouts - where you pause half way through tying two ropes together and ask yourself why the heck you are doing this...

Well, aren't you grateful by now you learned to ty ropes?

The comma splice does not only sound awful, it is als condemned.
(splice-free sentence??)
'It is condemned in The Elements of Style'
But the page is really instructive, zmjezhd.
(can't call you nuncle, being a naunt)