Grey card

The standard card reflects 18 per cent of the light hitting it, which sits close to the middle of a scene's tonal range. Film a few seconds of it under your actual lighting at the start of a session, then in the edit use that frame to set white balance and check exposure across every clip from that setup. It costs almost nothing and it removes the guesswork from matching shots, which matters most when you have two cameras, mixed daylight and LED, or a session shot across several days. Skin tones are the thing it protects.