Crockett & Jones are probably the premier British markers of shell cordovan shoes - truth be told, that's not a hugely competitive title. Compared to the bigger American makers like Alden or Allen Edmonds, it remains a pretty niche material unless used in made to order models.*
C&J use Horween cordovan, generally the best-regarded** widely available shell cordovan.
I've reviewed here three pairs of C&J shell shoes in their dark brown, whiskey and burgundy shades. They also offer black - I've only owned one black shell pair from C&J and unfortunately don't have any photos of it. It's not a cordovan shade that does much for me - it tends to lack the depth of colour of the others, and comes across as a bit military for my tastes.
Sadly I've also since sold the burgundy pair, so I can't do a full side by side comparison in the same light, but this should give an idea of what you can expect. Jesper from Shoegazing has written at length about the various pros and cons of shell shoes, so I won't repeat that too much, but worth a read if you are thinking about investing in some.
Dark Brown Harlech Boots
These Harlech boots have been with me for a while. This side photo in particular shows the colour variation that has developed if you contrast the side panels with the toecaps, though the effect is somewhat less apparent in real life.
I'd say this is probably the most versatile shade, and a very easy one to dress up or down as the context requires. If you're looking for the equivalent in Alden models, I think the colour name is just "Brown".
One of the benefits (but some would say downsides) of shell is how the colours develop over time - you can see how the leather has darkened at the point of the foot that sees the most flex, heat and moisture.
If you're particularly anal about exact colour matching on your shoes (and even the different panels on your shoes) then shell can be a risky purchase. You won't get the same uniformity of colour across the shoe as you see on the website models, even straight out of the box - see the link below for C&Js own product photos. If uniform appearance is a real need then these models are mostly available in calf leather too.
Dark Brown Pembroke for Ralph Lauren
A recent purchase, and another good example of the stark difference in colouring you can get in shell - compare the side quarters with the heel.
These haven't seen significant wear yet, and from some angles do demonstrate the "plasticky" look that many dislike about shell - to the untrained eye it could easily be confused with the kind of bookbinder leather that Church's and Tricker's use in some models. The quality of the leather show once it starts to develop a patina and the richer colours come through.
Burgundy Adelaide Captoe Brogues
I believe this pair was about 10 years old, made for Jermyn Street outfitters New & Lingwood. I loved this pair, and really only sold as they ran about half a size too big so never got a lot of wear.
The burgundy shell is an intensely vivid colour - to use a very over-worn phrase, it really does pop in the sunlight. It's difficult to describe until you see it in person, but the sheer richness and depth of colour of a well-worn pair of shell shoes is enticing and addictive.
The Alden equivalent shade is of course their most well known - Colour #8. It's their most popular colour, and pretty readily available on eBay (provided you can navigate Alden sizing).
You get hints of plum, aubergine, ruby, dark brown - it's very pleasing and I'm keen to pick up another pair in this material in future. Speaking purely subjectively it's probably my favourite of the three shades. This model isn't available RTW from C&J, though I suspect you could arrange a made to order version for a slight surcharge.
Whisky Pembroke Wingtip Brogues
My newest pair, these were a full-price purchase from C&J. I'd spent a long time waiting for a whisky shell pair to show up on eBay, but eventually, just bit the bullet. Almost completely unworn still thanks to the pandemic, but they are an exquisitely made, hefty pair of shoes.
I love the subtly contrasting piping along the top-line and lacing. If I was being pedantic about complaints (and I can, since it's my blog):
1) Really needed toe-taps adding - the tips of the soles very quickly started to show some really unsightly wear, a lot more than I'd expect
2) A lot of polish and general debris and gunk in the brogue holes. This was easy enough to clean out with a chopstick and a stiff brush, but did look somewhat untidy, particularly for a relatively expensive pair
The lighter shade of whisky does make some of the colour matching issues traditional from shell a bit more apparent (in other words, one shoe is definitely a bit lighter than the other) and like most tan coloured shoes it does overall have a slightly "flatter" appearance, though time will tell how much those colours develop over time.
* Edward Green currently have a small number of cordovan options available, including a lovely cordovan Galway boot. I've seen a couple from Tricker's in the past, though it's stiffer and glossier than C&J's cordovan
** "Best" is evidently subjective here, but have a look through r/goodyearwelt to get an idea of the rough pros and cons of the big producers of shell cordovan