Regulate brightness over video composite signal
Your camera's sensor is probably saturating in daylight so attenuating the video signal would just mean that everything looks grey rather than white. The only cure is to use a lens fitted with an iris to reduce the amount of light entering the lens. Better still, fit an automatic iris lens which uses the video signal to adjust the iris aperture to keep the video level constant.

It is also possible to use neutral density optical filters to reduce the light which would be cheaper to retro-fit but difficult to make adjustable.

As an aside, a composite video signal contains timing information (sync pulses) in addition to video brightness information, so attenuating the composite video signal would also attenuate the sync pulses - which is not desirable.