Yes, bell pepper is safe for a bearded dragon, and it is one of the most reliably tempting vegetables you can put in front of one. Red, yellow, orange and green peppers are all acceptable — the riper colours are sweeter and usually preferred — and pepper brings useful vitamin C and hydration to the salad. Its role, though, is garnish rather than foundation: chopped bell pepper belongs scattered over the staple greens a few times a week, adding the colour and crunch that persuade a reluctant salad-eater to start.
Preparing pepper the dragon-safe way
Wash the pepper thoroughly, since peppers are frequently waxed and sprayed for the human market. Cut away the stem, every seed and the pale inner ribs, then chop the flesh into pieces no wider than the space between your dragon's eyes. Seeds are left out as a precaution — they add nothing and are exactly the size to be swallowed whole — and the skin can stay on the pieces, as dragon jaws handle pepper skin easily. Serve it raw; cooked pepper collapses into a sweet mush with much of its appeal and texture gone. If your dragon is new to pepper, mix a little through a familiar salad rather than presenting a red pile that looks like a challenge.
How much, how often
A few chopped pieces across an adult dragon's salad, two or three days a week, is the sensible rhythm alongside staple greens such as collard or mustard greens. Bell pepper carries more natural sugar and water than those staples, so a dragon allowed to fill up on pepper daily is a dragon quietly training itself to hold out for the sweet bowl and skip the nutritious one — the pepper version of the classic picky-dragon problem. Bell pepper does not replace calcium dusting either; salads still get their usual light coating. For juveniles, a taste of pepper in the small vegetable portion of their insect-heavy diet is fine and builds the flavour familiarity that pays off in adulthood.
What to watch after serving
One soft stool after a generous first serving of watery pepper is common and no cause for alarm; cut the portion and it firms up. Repeated undigested pepper skin in droppings suggests the pieces are going in too large — chop finer. Never offer pickled, roasted-in-oil or spicy peppers of any kind: the bell is the only pepper on the menu, and chillies and seasoned jars stay in the human kitchen where they belong.
For an unfamiliar fridge item you have never fed before, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library, so the pepper-or-not decision is settled before serving.