Purple kale is an attractive vegetable that has a very robust cabbage flavor that is more intense than either black or green kale.

Its leaves contain semi-crisp texture that is chewy and goes soft when it is cooked.

They stems are fibrous and tough and need to be removed before they are cooked.

Purple kale has leaves that are colored a vibrant purple and are variegated in shades of green. (source)

So can bearded dragons eat purple kale at all?

Lets take a look at their nutritional data and find out more.

In particular, their acidic, calcium, phosphorus, water, protein, fat, sugar, and fibre content is of most interest when it comes to bearded dragons.

Another thing of importance is the calcium:phosphorus ratio as for every part phosphorus, there needs to be another part calcium if not more. From 1:1 to 2:1 is ideal as far as bearded dragons are concerned.

Nutritional value per 100 g (3.5 oz)
Energy 207 kJ (49 kcal)
Carbohydrates
8.8 g
Sugars 2.3 g
Dietary fiber 3.6 g
Fat
0.9 g
Protein
4.3 g
Vitamins
Vitamin A equiv.
lutein zeaxanthin
(63%) 500 μg
8198 μg
Thiamine (B1) (10%) 0.11 mg
Riboflavin (B2) (11%) 0.13 mg
Niacin (B3) (7%) 1.0 mg
Pantothenic acid (B5)
(18%) 0.9 mg
Vitamin B6 (21%) 0.27 mg
Folate (B9) (35%) 141 μg
Choline (0%) 0.8 mg
Vitamin C (145%) 120 mg
Vit E (10%) 1.54 mg
Vitamin K (671%) 705 μg
Minerals
Calcium (15%) 150 mg
Iron (12%) 1.5 mg
Magnesium (13%) 47 mg
Manganese (31%) 0.66 mg
Phosphorus (13%) 92 mg
Potassium (10%) 491 mg
Sodium (3%) 38 mg
Zinc (6%) 0.6 mg
Other constituents
Water 84.0 g

source wikipedia

As you can see, purple kale contains a huge amount of water and acidic content, a lot of phosphorus, a hint of salt, Its calcium:phosophorus ratio is about 1:1.

This means that purple kale is not good for them unfortunately. It is too high in acidic and water content and will give them diarrhea if fed to them.

There are better foods to be fed to a dragon.