GFC has been pretty clear at what stages changes can and cannot be made. I don’t know if they’ll sell an additional tent itself in the fabric color of your choosing, but I’m sure that would be some ways out given their production planning and materials ordering.
Assuming the person you bought it from made it clear that options were chosen and changes couldn’t be made? There are obviously some shortcomings with customer service at GFC which have been extensively discussed on this forum, but the fact they even let people sell spots seems to be a benefit for many people.
Either way, hope you enjoy your camper and have a safe trip up for install next week!