[susan53]

If I've understood correctly, what you mean is that you're not trying to say :

Example A

A : Which is the one you like?
B : It's the red one.

but :

Example B

A : Which one shall we buy?
B : The red one is cheapest
A : OK, the red one it is!

In example A the speaker is simply identifying the object. There's no reason to front the topic, because there's no emphasis. Unless B happened to be Yoda from Star Wars (like that he speaks), the emphasis that the fronting in The red one it is would gives would sound silly.

But in example B the gloss could be - OK, The red one is the one we will buy. Here - The red one = S. It is here "substitutes" (ie means) is the one we will buy.

However, we could just have easily had said The one we will buy (=S) is (=V) the red one (=C). As you said, the other word order was probably chosen for emphasis : the speaker sees "the red one" as the topic of the sentence and fronts it. In this context emphasis is relevant. This parallels with sentences like Joe, his name is or An utter idiot I felt (examples from the Communicative Grammar Leech and Svartvik section 427).

It seems to me that the reason the non-emphatic order (It's the red one) can't be used, as opposed to the equally non-emphatic The one we will buy is the red one is because of the potential confusion with the purely identifying meaning in example A. As ever, it seems to be the difference in meaning of "it is" in the two examples which causes the difference, rather than any purely grammatical rule.

But groping in the dark a bit I am. Other ideas other people have perhaps -??