Your question is quite clear. It is known that G&M use caramel for colour consistency, no doubt about that, if you look at the bottles you may notice that the colour is practicly the same, that nice famous brown colour.
But if Macallan should buy back any whisky from G&M, they probably would buy the casks back again, why? Well it's quite simple, if they would buy back the cask, then they have the full insurance that nothing has being done with whisky, but however if they buy the unlabled bottles back, well that's another matter, they don't know what's happend with it, they didn't have any eye on the production at that time, so there's no control or what so ever, so if you put your own label on it, then you take some risks here. I don't think that would buy any unlabled bottles at aal, occasionally what happens is, if they want some their own rare vintage bottles back, with their own label, they would try to buy them back again, but only if it has their own label on it....
I hope Iain that this answer your question.