The reason the “scales” waistcoat has been given its nickname is because of the weave of the fabric itself.
In the show, the waistcoat usually appeared to be solid gray:
Sometimes it took on a purple-ish hue, too.
Even on Blu-Ray, seldom were hints of the fabric’s actual texture and weave visible.
In fact, during the course of my research, I only found a single instance of the “scales” fabric being clearly visible in the show.
Fortunately, publicity photos for the show were usually very high-resolution, and the “scaley” texture of the fabric is much more easily observable than in the show itself.
It’s also easy to see why the waistcoat sometimes took on a purple-ish color.
Observe that the “scales” pattern wasn’t simply printed onto the fabric; it was actually woven that way, producing the textured effect.
Also observe that the fabric was oriented with the rounded part of the “scales” facing upward on the waistcoat.
If you were fortunate enough to have visited the Doctor Who Experience before it closed and saw the 11th Doctor costume display there, you may recall that the texture and colors of the fabric are easily observable in-person.
As “The Ginger Doctor“ and Daniel Pawlik discovered, the fabric itself is a wool/silk blend and available in a variety of colors.
“The Ginger Doctor” also has a beautiful high-resolution scan of the fabric on his blog, accompanied by a description of its strange properties. Check it out via the link below:
https://thegingerdoctor.wordpress.com/2017/04/01/tipping-the-scales-eleventh-doctor-nirvana/
On a final note, one can’t help but wonder what the intended purpose for this strange fabric was and/or is.
Some sharp-eyed fans have subsequently observed it – and its alternately-colored companions – on other (non-Who) costumes, but I suspect it may have originally been intended for upholstery and was “adopted” for apparel use, as is often done in the entertainment industry.
Not long after I began researching this waistcoat, I went to lunch with some friends and something about the restaurant booths caught my eye …
It’s nowhere near an exact match – not by a long shot – but the essence of it was certainly familiar.
Now, just file that away under “things you can’t unsee” and try not to look like an eccentric weirdo photographing restaurant booths while hanging out with friends.