What does the mother of the groom wear to a barn wedding?

I am sure I am stressing over this way too much but what can I wear to a barn wedding at dusk in September? Our son will be wearing a tux and she her wedding dress. You will walk down to the barn on white rock so I know not to wear heels and I haven’t worn them anyway for ever but I believe I do need closed toed shoes. They will have a pizza bar and a taco bar etc afterwards..maybe that will help you with your answer. BTW their color is olive green which I love…And absolutely nothing white or black right?