Because BRIO 500/501 streams with HDR in real-time, you may experience video that is lighter or darker than expected in scenes with heavy contrast (ex. a lot of white or light colors and/or darker colors and tones). The camera may adjust the scene in a way to compensate for the contrast extremes which can cause the entire scene to become lighter or darker.
One thing you can do is introduce more natural back and side lighting. This will trigger the HDR feature to make the appropriate adjustments. You can also try sitting closer to the camera — try 30 inches and adjust closer or further to see what works best.