Tinting gelcoat can be a pain,but if you have a good boat repairer in your area,I would have a talk with him about tinting the gelcoat for you,much the same as a car painter tints paint,probably just needs a touch of black and/or blue in it to bring it to the right shade,whites can be a real b****h to get right,maybe you need to contact the manufacturer of the tub,and get the correct colour for the job,failing that,revert to plan B,turn out the light,or wear a mask when you take a bath...