I'd repeat, the rule is : the ball shall be white unless otherwise agreed. NO agreement- white ball must be used. - a Good rule It is important that there is a hard and fast fall back position. How would you like to adjudicate the debate between two captains at the start of every match ? and two umpires with different preferences !!! The captains have a choice- agree a colour - white or otherwise, or one of them can decline to play. His only " valid" reason is that he considers it unsafe to play even with a white ball. Don't think that argument would be acceptable in normal daylight. Under lights, I'd presume that the association has accepted that the ground has adequate lighting, so that would hardly stand up. Real difficulty is if some lights are out- a not uncommon occurrence- then there is room for " opinion" .- or if it is a dark day. Called " bad light stopped play" Light meters, anyone? Umpire can't see ball - change umpire if possible. No other umpire? Abandon match? Agree it is unsafe to play? The important bit is that the ball MUST be white unless otherwise agreed - a standard.