Red Cabbage is also known as the purple cabbage, red kraut, or blue kraut. It has dark coloured leaves. It changes colour according to the pH value of the soil due to a pigment belonging to anthocyanins. It can be found in Northern Europe, throughout the Americas, and in China.
So can rabbits eat red cabbage at all?
Lets take a look at their nutritional data. In particular, their acidic, phosphorus, calcium, sodium, sugar, and fat content is of most interest as far as bunnies are concerned.
Energy 122 kJ (29 kcal)
Carbohydrates
6.94 g
Sugars 3.32 g
Dietary fibre 2.6 g
Fat 0.09 g
Protein 1.51 g
Vitamins
Vitamin A equiv.
beta-carotene (0%) 2 μg
(0%) 20 μg
Folate (B9) (6%) 24 μg
Vitamin C (41%) 34.4 mg
Vitamin K (45%) 47.6 μg
Trace metals
Potassium (6%)
source wikipedia
As you can see red cabbage contains a hint of fat and a little sugar.
This means that red cabbage is fine for rabbits to eat and makes a fine supplement for their hay diet. However it is known to cause them to have gas if you give them too much, so twice to three times a week in small amounts should suffice. Just make sure it isnt their main food.