The first time I saw cabbage used as an ornamental was about 25 years ago, and it left an impression on me, but not necessarily a good one. The landscapers at Seabrook Island, where I worked, had planted many of the pink variety (probably Brassica oleracea var. capitata 'Osaka Pink') en masse with solid yellow pansies. It seemed few beds at the resort were neglected from this combination. They were planted in a rigid grids like so many pink soccer balls lined up for practice. What I knew of horticulture then was not much, but I knew this plant looked more at home on the kitchen counter than in the landscape.
Fast forward to today, and I am still no fan of most ornamental cabbage. However, there are a couple of members of the Brassica family that regularly have a place in my winter garden. One of these is Redbor kale (Brassica oleracea 'Redbor'). I like that it grows taller than other Brassicas, has a very interesting texture, and I love the smoky purple color. I once had another horticulturist tell me that as soon as you see the flower stalk on any Brassica then it was time to pull them out. Last spring I let mine hang around long enough to bloom and the sulphur-yellow flowers were a nice compliment to the dark foliage, and I was very happy that sometimes I don't listen to what other horticulturists say.
Perhaps my favorite ornamental Brassica is giant red mustard (Brassica juncea var. rugosa). The large puckered leaves are a shiny burgundy and the stalks and ribs can be a bright acid green. I use to always pair these with solid yellow, faceless pansies, but the past couple of years I have been using Delta Tapestry that contains some of the same colors as the mustard mixed with shades of yellow. Giant red mustard will also bloom with the same type of flower as the kale, but I don't find it as reliable.
Both of these plants need full sun and put up with what kind of winters we have here in zone 8, but I know they can take colder. During the warmer days of fall you will have to keep them moist, but once they are established and temperatures cool, little supplemental water is necessary. Last year we had some heavy-for-our-area snow that temporarily mashed down the mustard, but did not seem to phase the kale. Even though Redbor kale and giant red mustard are sold as ornamentals, they can both be eaten, but I prefer another member of the family for that.
I usually find my favorite edible Brassica between two halves of a bun keeping company with some barbecue. I like my Q North Carolina style, which is usually made from slow cooked pork shoulder, finely minced, doused with a vinegar based sauce, and it must be topped with coleslaw. Anything else is just another meat sandwich.