Interesting. I've always been under the impression that the newly-distilled spirit was colourless, and the colour developed from the casks it was matured in.
What do the distilleries that don't use old sherry casks use instead ?
To add to
@PK99 's post...
Yes, the distilled spirit is clear (or at least should be!)
The cask must be oak and traditionally sherry casks were in abundance because sherry was shipped in bulk from Spain & Portugal.
Bourbon casks are also used as they are relatively cheap because by law, they can only be used once. (A senator who owned large oak forests was influential in passing the law!)
Other than that, casks are made and charred/burned on the inside and are used.
Other previous ingredients can be wine, port, brandy, rum etc, although depending on the product it may not be suitable for maturing, instead used to "finish" - taking a mature whisk(e)y and putting it into a different type of cask to add extra dimensions of colour and flavour.
To add to the complications, there are, of course, lots of different types of sherry, or wine for example. And the casks have a limited life span, or more accurately, the previous contents become less significant with every use or with the passing of time. So a "First fill" port cask may be quite unsuitable for maturing a whisk(e)y, but after used a few times for "finishing" a whisk(e)y it may well be suitable.
A distillery here in Holland (technically Friesland) wanted to be the first Dutch whisky distillery and under pressure from another company rushed out a batch of red wine cask matured whisky to a lot of acclaim. I missed out on the first batch, but got a couple of bottles of the second.... absolutely vile!
That's a serious problem for new distilleries - they really have no idea how their whiskies will develop and mature.
Just a note on colour. As
@PK99 pointed out, caramel is allowed.
Distilleries will argue that it is flavourless and is used only so that each batch will look similar to the next. Arguments can be made that the visual presentation can influence people's expectations and thus, the taste.
I had a 21 year old whisky that had barely a tint of yellow to it - it was almost water-like (in colour!) Many people could not believe it was so old as they "expected" such an old whisky to be much darker.
Distilleries are very reluctant to disclose this to their customers. Distilleries market on the "purity" of their product, so a colouring undermines that message.
However, the good old Germans insist that all additives are highlighted on food/drink labels, so labels in Germany must have "Mit Farbstof" if the caramel is present.
Something that is not highlighted on this side of the Atlantic is the fact that Bourbon is the "purest" of the whiskies due to the fact that absolutely nothing can be added!