Capturing the best colors of your bearded dragon depends on a few issues, not just lighting, though lighting does play an important role. Quality of camera and all that plays a part.
Anyway, know your camera inside and out is a first good step. Read that manual that came with it. It will tell you all the capabilities of your camera, including best focal length, if it has settings for low light etc.
Sunlight is good but it can also be blinding and cause harsh shadows etc. Learn if your camera has a fill flash...so harsh shadows etc, are removed. Slightly diffused sunlight would be better than direct overhead sunlight, maybe near dusk or dawn, were the sun isn't as harsh, but again, watch the shadows.
I am no where near an expert on taking pictures...I consider myself good if the snake or lizard I am trying to take a picture of stays still long enough....and the batteries don't die just when I want to take that 'perfect' 'cute' or just plain cool picture.
Light from all around works best, to prevent harsh shadows. But not too bright that it will wash out all the color. A light overcast is probably the best time to take pictures, shadows are softer but there is still enough natural light to get good pictures.
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PHLdyPayne