Greyout

[1] It is a precursor to fainting or a blackout and is caused by hypoxia (low brain oxygen level), often due to a loss of blood pressure.

A greyout can be readily reversed by lying down as the cardiovascular system does not need to work against gravity for blood to reach the brain.

[2] This is the reverse of a redout, or a reddening of the vision, which is the result of negative g-forces caused by performing an outside loop, that is by pushing the nose of the aircraft down.

Surprisingly, even during a heavy greyout, where the visual system is severely impaired, pilots can still hear, feel, and speak.

Roller coasters are unlikely to have high enough negative g-forces to induce redouts, as most low-g elements are designed to simulate weightlessness.