THE Rainbow Cup will be used to trial red card replacements, captain's challenge and goalline drop-outs.The Dragons get the new tournament under way against the Scarlets next weekend and they will have to get to grips with three law variations.The Cup will mirror Super Rugby in experimenting with changing red cards from a dismissal to a 20-minute spell off the field, with teams then allowed to replace the offender with one of their substitutes.Captains will also be given one challenge per match to review tryscoring and foul-play incidents, with the scope to look at any of the ref...