Greying out of vision 'during' (not after) a hard cough fit (after inhaling). this greying out becomes white patches in dark during coughing. normal?

Yes: This is a normal transient reaction to a hard cough or other types of valsalva maneuvers (sneeze, hard bowel movement, etc). As long as the symptoms resolve quickly, there is nothing to worry about.

Yes: Coughing can cause momentary increased pressure in eyes that can slow blood flow and cause these symptoms. Also, can 'activate' vagal nerve which also slows blood flow.