bumping!
I searched around for you for a while, and it's a bit of a stretch but the closest thing I could find is this:
https://www.mangago.zone/read-manga/the_forgotten_princess_wants_to_live_in_peace/
There's a scene ~chapter 59/60 where a girl is called the Lavender Lady and is head to toe in purple/they think she's the duke's fiance. It matches up with the scene you described somewhat.
OMG I recognize this title! This might have been it, thank you so so so so soooooooo much!
Turns out it IS the right manhwa! Thank you again!
Yay! I'm always glad to help
Hi! So at this point I wasn't sure if this was isekai, but I think it is.
The only scene I can remember is that there was a girl whom people call "Lavender Lady" simply because she wears lavender color dresses and accessories head to toe, and she has feelings for the Duke (?). And since she is called Lavender Lady, the nobility assumed that she is the fiancee of thje Duke. But the female MC is actually the real Lavender Lady and the Duke (?) likes the female MC and said something along the lines of "I know what you are doing, stop using the name Lavender Lady to get close to me" or something.
I have a feeling the Duke is actually the MC's father and I might have been mixing some details up but I am 100000% sure this wasn't like, in*cest or something. It was either the female MC is the Duke's daughter and the fake Lavender Lady wants to be the Duchess, or the Duke likes the female MC and wants to put fake Lavender Lady to her place by having the female MC reclaim her rightful title as the Lavender Lady.
Please, I know my summary is pretty vague and messy but if you are familiar with a story with the same scene please let me know.
Thank you!