I have no direct knowledge about dark hulled Jeanneaus but in my experience with my own dark hulled boats, I found that the boat tended to get coated with salt as water flash dried on the topsides much easier than my white boats. This meant that I was scrubbing the hull more frequently and that the fenders were more likely to abrade the hull where the fenders rolled back and forth on the hull. As a result, I was waxing the hull three or four times a year to keep the boat looking bristol.
Although I believe that dark hulls are a lot more maintenance I just bought a boat with a dark blue hull. That said, at some point I intend to have the boat painted white.
Being the former owner of a dark blue hull, I perceive that there are other issues that would suggest the avoidance of a dark fiberglass hull. Unless the hull was specifically and originally layed-up as a dark hull and has ***sufficient/extra thickness of matting underlayment*** of the gelcoat, there will be eventual additive thermal deformation and dimensional changes of the surface as evidenced as "print-through" of the fiberglass roving, etc. or surface wrinkling. Most of this print-though may be cosmetic but some other dark hulls I''ve noticed seem to have accumulated/additive structural deformation where the dark hull intersects a bulkhead and/or chainplate attachments, etc. ---- very visible deformations. Simply take a walk around any boat yard and especially look at the older dark hulls (especially those constructed with fiberglass roving) ..... most are wrinkled and have ''print-through'' and other surface deformation. As one with an extensive stress analysis background, I perceive this is not good!. I further perceive that the base polyester polymer on a dark hull continues to additionally cure because of cyclic high thermal stress (constant heating and cooling) thus causing the print-through, wrinkling, and perhaps even structural anomolies. I understand that in the past, several manufacturers of cored composite hulls have had many problems (weakened laminate) with dark colored hulls - Niagara, etc.
But they are Pretty.......... until you eventually paint them white.
My first boat was a beautiful dark green. I rubbed against a dock, putting some scratches in the color. I told myself then, I would never buy another boat with a colored hull. My next boat was and is navy blue. I just couldn''t resist, they are just so good looking. (I also bought 10 bumpers to avoid any scratches) Jeff is right, i have to wash the hull down every other week and wax a few times during the season or the salt makes it look dirty. But i think its worth it.