Requesting guest wear specific colors to your wedding

This is good question and I'm glad I came across this video! My sister is getting married next month and requested I wear something in her color scheme. I, however am on the fence about it because I have not received an invitation, am not in the wedding but was just told about the event. Her other sister is in the wedding and I just feel like an afterthought. So the dress I have picked out does not go with her bridesmaids or her colors at all, but I feel I should be able to wear what I want since she has not included me in any of her plans to begin with. Nothing at all to "get back at her" its just what im comfortable in. Also I'm not going to go out and spend more money on a dress for one evening.
P.S. I am so sorry for making this sound like a therapy session. But your short was so kind and you seem like someone whose seen it all!
Be blessed! One Love 🤘🏼💚