Typically dragons will 'fire up' when exposed to high levels of UVB and UVA light (ie natural sunlight, high uvb/uva outage bulbs, such as mercury vapor bulbs and higher rating florescent tubes (repti-glo/repti-sun 8.0 or 10.0)). A well balanced diet with colorful vegetables/friut and flowers added in can also help with showing off of colors in the dragon.

However, not all bearded dragons will have alot of color, pretty much those who are selectively bred to produce bright colors will be able to show them off. A normal grey/brown/black dragon will not start showing any colors it doesn't have, not matter what is fed or done. (short of using water based pigments directly on the dragon itself, or food dye). In direct sunlight, even this 'drab' normal dragons will lighten up, usually because they are warm and don't need to absorbe more heat, so their color lightens up (lighter shades don't absorbe as much heat as darker shades. Wear a black shirt in the sun and see how long it takes for you to get warm, then wear a white shirt and notice the difference).
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PHLdyPayne

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