Your primary focus is on your dress, what your fiancé is going to wear and the clothing for the wedding party. However, there are things to consider regarding your guests as it is YOUR day and some things aren’t appropriate.
White: Wearing white (or even a shade of it) should not be done which goes without saying. Unless you have a black and white theme or specifically request certain people to wear it, doing so is unacceptable.
Black: No one wants a color associated with funerals and grief. If someone feels more comfortable wearing a darker hue, a shade of blue or dark green is fitting. After all, it’s a celebration and colors should be cheery and upbeat.
Sparkles: Guests might want to add a little accent to their outfit but going overboard is disrespectful. That should be saved for their own wedding day as lighting and photography will catch the glimmer and outshine you even if not intended.
The best way to avoid any mishaps is to include a dress code on your invitations. Even if you say semi-casual, your guests should dress up rather than down. To avoid any confusion, include an option where you can be contacted with questions.