There are so many eco-friendly options for Halloween costumes. If you are a crafty bride, you can create your mask and gown yourself. This allows you to choose your own sustainable fabrics and you can control the overall cost a bit more. If you don't think you can get a costume together in time for your big day, you can shop at costume rental shops or vintage stores for classic designs that are also affordable.
If you don't want to transform yourself completely, wearing a normal bridal gown with a few masquerade ball touches such as an organic cotton mask or ornate peace silk gloves will still make you look like the eco-chic belle of the ball.