Usage of "same" vs. "selfsame"

Macmillan has:

selfsame ADJECTIVE [ONLY BEFORE NOUN] FORMAL . . .

used for emphasizing that something is exactly the same as another thing

He asked me the selfsame question.

So you wouldn't say 'the colours orange and amber are nearly the selfsame' or 'orange is the selfsame as amber'.

You'd use it when adding emphasis:

This is the selfsame room we had when we stayed here 25 years ago!

But probably not for the less dramatic news:

This is the same room we had when we were on our way to the Grand Canyon last week.

I wouldn't agree that the usage is formal, but I would say it's not used too often in colloquial speech.

The essential difference is that while "same" can be used in relation to things that are identical in appearance and other properties (same genus), "selfsame" will denote one thing reappearing in other circumstances.

This car has the same engine as in the one my brother sold last year!

This car's engine is the selfsame as in the one my brother sold last year!

In the first case it will mean the engine is the same model, but doesn't imply it comes from the brother's former car. In the latter case that specific engine was moved from brother's car to the one in question.

In many cases the two will mean the same thing (often 'the same' will apply to that particular instance, "same place" simply used without the extra emphasis of "not a different instance of the same kind of place") but "selfsame" won't be used for "different, but looking the same".